Candice Wing says

I’ve met a good many mature men looking for affairs and divorced men looking for a second wife. None of them have said – “oh dear me, my wife is old and fat and thus unattractive and therefore I feel compelled to seek a younger and therefore more attractive option”.

Source: Candidly Candice

She goes on to list real reasons men have told her for separating from their partners

• Wife does not want to have sex with me or wife does not want to have enough sex with me.
• Wife does not like me and does not have sex with me.
• We are not compatible and I am looking for more than just boring sex.
• Wife is not affectionate.
• Wife is boring in bed and generally boring.
• Wife is a cranky harpy.
• Wife is lazy and boring with poor grooming and presentation.
• I (or wife) want to divorce.

You can read the whole thing yourself, and if you do you'll get her simple one-paragraph explanation of why the vast majority of men remain perfectly attached to their partners.

Candice has been writing a lot about her own experiences sex, love, and aging. This is another great post along those lines.

While, sure, some people (not just men) really do lose interest specifically over their partner's looks, it happens at any age. And if it happens at any age then emphasizing one age over another is just stereotype reinforcement.

Meanwhile the other reasons you list are much more plausible, particularly for very long-term relationships. Although, hmm, now that I'm thinking about it even that shows up more predictably at certain points in a relationship than at certain ages. For instance I seem to recall there's a spike in divorce rates at the 21-22 year mark whether the couple marries in their late teens or mid 40s. And if you just think about it for a minute, if some people in their 40s find their flames going out while others in their 40s find themselves igniting, then age probably isn't the cut-off factor young people, hack novelists, and pop social scientists keep claiming it is.

Either way I agree with Candice that it's way more complicated than the popular but too-pat stories about husbands leaving because their partners "lose their looks" post-menopause. In fact it's so complicated it might not be happening for specific age-related reasons at all.

Sex educator Julie Sunday offers the following pithy summary of an analysis by Professors Kathrin Stanger-Hall and David Hall of state sex-education policies and rates of teen pregnancy and birth.

Sex education matters, yes, but access to services is more important. Teens do not have sex for the purpose of avoiding pregnancy--they have sex because sex is fun. If adults and policymakers want teenagers to use birth control, they will--but we have to teach them how to use it and help them figure out how to get it instead of erecting [heh] insurmountable barriers to keep them from avoiding pregnancy and spreading STIs.

Teen sexuality is like the Titanic--the ship is definitely going down. We can either play music and pretend we're not sinking or provide life jackets and get the people off the ship already. Considering that the House's recent budget proposal included renewed funding for the terrible, horrible, no good very bad Community Based Abstinence Education program (Read: federal government gives money to religious organizations to provide "education" in public schools and make cheesy PSAs), this country is still letting the ship sink without enough lifeboats for everyone.

Source: How to Have Sex in Texas

It's an interesting, sort of back-handed twist on the Titanic metaphor but I think that's about right.  The idea, incidentally, isn't to make birth control and sex safety materials available so that teenagers (or anyone else) will have sex, it's so that those materials will be available if or when they do.

So over the years you might have noticed that some people stereotype the owl-poop out of whole classes of people. It's not always malign or dismissive. Sometimes stereotyping can arise from positive or shared experiences with individuals that... can get spatula'ed onto everyone who matches the "category" in question. Which might be fine if the category of persons all really were as a) ideal as claimed, and b) as interchangeable as claimed. Oh, and c) as willing to be homogenized in someone else's mind with all the thousands or millions of individuals the onlooker imagines they resemble.

When one does this -- when one opines that "oh, 'all Africans' are so beautiful and accepting" (based, say, on your Peace Corps experience in a single village in a country continent (almost) bigger and more populous than all of North and South America put together) or "Asians are my favorite students" or "ooh, librarians are hot," etc. -- one may have nothing but the best intentions but one is still engaging in objectification.

One can be no less objectifying even if the category one is drawn to is more often negatively stereotyped. In fact, one can be no less objectifying even when you yourself are a member of the negatively stereotypes category.

I mention this first because one of the most controversial forms of objectification revolves around sexual attraction. And second, because I stumbled across a pretty cool post by new-to-me male fat activist Brian of Red No. 3 who does a very cool job of distinguishing attraction from objectification.

So, I’ve noticed some of my fellow male fat admirers throwing tantrums when women object to be sexualized without consent. These dudes whine about how the women are telling them aren’t allowed to find fat bodies attractive.

Cut that shit out. Like now.

No one is out to confiscate your boners. Sexual attraction to fat bodies is totally awesome. There may be people out there who want to shame you for your sexuality, but its not these women. So, by all means, holster your outrage and listen up.

The issue these women are complaining about isn’t sexual attraction. They are asking to be treated with respect and dignity. Try not to be shocked at this stunning request. You still get that be sexually attracted to fat women. Just, maybe respect them.

And actually, strike that maybe.

Source: Red No. 3

It's definitely worth reading the whole thing. It's ok to be attracted. It's just not ok to forget the who who always and necessarily goes with your what.

Actually, if I can briefly bring in another contentious term, we're all entitled to our preferences. In fact try not being! We are not, however, and never can be entitled to the favors or affections all or even any individuals who happen to embody our preferences.

The rest of Brian's post is similarly sharp and it would be great if you just went and read the whole post. One thing I really appreciate is the way he invokes both altruism and self-interest.

This is especially important for fat women who already live in a culture that conspires to desexualize them. They often find themselves in scenarios where they are told to choose between never being desired sexually or always being objectified sexually. That’s fucked up and wrong. You should be able to know that by just basic empathy, but I’d submit that as fat admirers its in our interest to combat thin privilege and male privilege. Not just because standing with our current or prospective romantic and sexual partners on issues of basic human dignity is the right thing to do (though that really should be enough), but its in our self-interest, too. Those restricted options women face impact us, too. We are being taught that our sexuality is wrong and that if we act upon it that we are deviants. We are told we don’t deserve to open, loving relationships with partners we are sexually attracted to. We are told we shouldn’t date them because they are “unhealthy”. We are told there must be some defect that causes our sexuality. We are being denied the opportunity to embrace our sexuality in the ways men with conventional attractions take for granted. The women who complain about objectification of fat women aren’t trying to take away our sexuality, they are trying to fight for it! We should stand with them and resist those who tell us to sexualize and objectify fat women because they don’t deserve better and we don’t deserve better.

This is just brilliant. When we judge and objectify we subject ourselves to equal objectification and judgment and consequently we reduce ourselves in the eyes of others.

And this is a universal point. Brian ends his post by opening his point

Oh, and if you’re a dude who isn’t a fat admirer, feel free to take the word “fat” out above and it apply the same to you because we all know you dudes do this shit, too.

I'd just add, finally, that the likelihood that it's men who get called for objectification is more an artifact of prior dating conventions than something (stereotypically!) innate to men: as more women take the initiative in dating, as more and more women continue to ask rather than wait to be asked, it'll be easier to notice how objectification tends to be more of a human characteristic than a gendered one.

Just how weird is it that nobody seems to be saying crap like "well, those boys had to have done something to get themselves raped." Nobody seems to be saying crap like "well, after so much teasing you can't really expect a horny man to control himself. Nobody seems to be saying crap like "well, they only 'cried rape' the day after because they regretted what they'd agreed to do the night before." And you sure don't seem to be hearing anyone brassing on about the need for awareness classes or self-defense classes or what-not-to-wear classes or 'don't walk alone' classes for boys. Not where the expectation is on boys to be on the defensive, to be perpetually vigilant, to be sure not to go around "asking for it."

You don't hear anyone opining that "sure, they're a little young, but since they'd have been 'giving it away' for nothing before too long anyway there's no real harm done."

Seems kind of funny to me, you know?

Kind of a paradox, really.

Of course there's a reason you don't hear any of that in the Penn State case.

It's because all that crap is an unreasonable expectation to impose on victims of sexual predation.

This evidently doesn't become clear when victims are women or girls. 

So however horrifying the Penn State case might be, or the Boy Scouts cases, or the Catholic Church cases, or the Republican congressman cases, it seems like there's some kind of teachable moment there.

Know what I mean?

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I gotta back up here and repeat something I mentioned only in passing above.

Nobody seems to be giving this guy Jerry Sandusky a pass for "doing what comes naturally."  Nobody's tisk-tisking about how he was just "thinking with his 'little head.'"  Nobody's going "well what can you expect, a man can only handle so much temptation!"

Not the way they'd typically give him a shrug if it had been the more typical "coach treat:" cheerleaders.

Another kind of paradox, eh?

Of course there's a reason you don't hear any of that about this guy Jerry Sandusky.

It's because all that crap is an unreasonable pass to grant perpetrators of sexual predation.

This evidently doens't become clear when a perpetrator's victims are women or girls.

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And I gotta touch on one more thing I almost completely glossed over above.

Nobody seems to be saying "those boys have had their precious jewel flowers taken from them."  They're not saying "nobody will want them now."

Which is kind of odd because, you know, when <em>people</em> are sexually assaulted and raped it generally has kind of a negative impact.

Another one of those paradoxes, only this one lands harder on boys and men in the sense that we have approximately zero social scripting for helping them work through that kind of violence.