Edited for more descriptive headline:

On one of the other threads here about maintaining T levels with diet, exercise and lifestyle, the guy was able to DOUBLE his testosterone in 90 days. at art of manliness

When you look at how the average LTR progresses, single dude with peak personal T meets girl, girl gets tingles and just can't explain it. But then life goes on. A year later they are talking about a future, which for a man often means focusing more on stability than opportunity at work, mortgages, kids, savings, debt, etc. The stress is a cause of increased cortisol levels that start to eat away at his T level. Having a good woman balances it out...or at least should.

From her perspective, she's already landed him, so she has let up on her closing game of being a shining light of peace and welcoming, that mixture of personal cheerleader, cook, masseuse, and whore that makes girlfriends awesome.

One of the first things to go for him is the gym, and he starts eating dessert again, having the odd extra beer, and he deserves it, because work is stressful now that he has to play the long game. He's less cut, looks "content," which just means fat. He's losing sleep because of stress, or just tstaying up to binge watch HBO series together. She isn't really a cook, why should she be, she has a masters degree, and few girls are these days anyway. He starts eating crap at work, skipping his protein blast breakfast, getting a sugar habit again. All the while, his testosterone levels are plummeting (see link above).

It's 2 years into the LTR at this stage, and he isn't the hunk she signed up for, but she loves him now etc. Except one day, she's out with friends in a bar, or takes up yoga or goes on a health kick, whatever, and the switch flips. The tingles are gone. She doesn't know why, all the pieces are there, but it's just not the same. When they argue, he seems neurotic, paranoid, even shrill. He's just not as much of a MAN. It's hard for her to respect him anymore, she wants to, but she doesn't know why it's just not there. She doesn't want to do it, because she knows how it goes, but she relents and releases the hamster.

What he doesn't realize is that his cortisol levels have been through the roof for 6 months, (and now he has the T-level of a 90yr old man) because the stress of work has just been compounded by the threat to the stability of his home. He smells like failure and can't put his finger on it. He's freaking out, and doesn't know what to do. He has become more beta than when she met him, and it broke the deal.

If her SMV is still high, she leaves him for some guy who wears a baseball cap and a tank top, and the man who was nuts about her is blindsided and loses his mind. Sometimes he ends up reading RP.

Whose fault was it? Both of theirs. He should have plated her, never depended on her, and never let himself go. But she let him down too. All those things that girls do that can make a guys cortisol level low and keep his T-level high are what 1950's housewives did.

If a woman wants to hang on to a man and continue to be attracted to him, in exchange for his support and all the stuff a man does, she needs to do a few things. Feed him like a lumberjack (protein and high fat, paleo), make his home a castle that he returns to as a conquering hero, fuck him like she is one of his french girls, and make sure he is always building, cutting, lifting, working out or being physical, don't throw a fit when he goes out with single friends, and especially if they end up at the 'rippers.

It's just that simple.

TL;DR: Women lose attraction to men when the guy's T-level drops below the level it was when she met him. It drops for a lot of reasons, some of which she can help. In a LTR, a woman can help to keep a man's testosterone levels at the level they were when she met him by studying 1950s housewives and adapting them to modern life.