Obligatory "I am 12 and what is this"
But seriously, at age 29, I am woefully unsuccessful in romance and have no idea what I'm talking about, and I don't feel that I can contribute to this discussion as well as some of the wiser, more experienced people here. I have not yet been broken by the world. So I'm just spitballing a little bit. I'm in no position to offer advice to anyone, so I will not do that.
Disclaimer: I am religious and believe the following things as assumptions, so take everything I say with a grain of salt: (i) divorce is not acceptable except in cases of adultery, (ii) sex before marriage is not OK, (iii) artificial birth control is not OK, and (iv) husbands and wives are duty-bound to serve each other. I just want to get that out of the way before saying anything else, because it shapes how I think about things.
Losing attraction for my future spouse is one of my biggest fears and a major reason why I've been cautious about getting involved in romantic entanglements. I often look at older men and their wives and I ask myself, "Would I trade places with them?" Very often, I think to myself, no way in hell. I see hippopotamuses with bob-cuts, and that makes me want to nope out of the marriage path real quick. There is no problem with getting older, getting stretch marks, getting crows feet, but some people really do let themselves go. Call me a sexist pig, but I wouldn't want to be with someone who just puts in no effort to look good. Don't get me wrong, I am far from perfect as is, and I looking pretty homeless right now, and I pretty much view myself as un-dateable at this point (but I'm working on it).
I'm also super fearful of being with a woman who loses her sex drive. That would be agonizing and frustrating to break out of.
In light of my fears, my game plan is to wait until I'm older, am done with grad school, have a ginormous income and a lot of money, and then go for a younger woman. I'm thinking that when I'm 36 I'll go for a 25-yr-old. That way, I'll have a lot to offer, and she's younger and therefore will have a longer sex life, and there isn't much biological pressure to have kids ASAP because fertility issues are not yet present. Preferably someone from a family-oriented community where the women value child-rearing, cooking, cleaning, family, etc.. I know I'm a pig for this, but it results in the best situation for me, so, yeah, sorry not sorry. And she's taken care of--it's a win-win for everyone. I play the long game and I'm overly cautious of bad marriage outcomes.
However, in the event that my wife stops taking care of herself or loses her sex drive... At that point I'd consider which of the following two situations we are in: (1) She's not keeping up because of something about me, and (2) She's not keeping up because of something about her. My first step would be to figure out which situation I'm in. Then, if we are in situation (1), I would try to get her to tell me completely and honestly the things I need to do to improve (probably easier said than done). But if we are in situation (2), the problem is on her end and there is literally nothing I can do. She might just be depressed. She is afflicted by this external thing, just as though she had cancer or some bodily disease. And I would suck it up with her; she's hurting, so therefore I am hurting. For religious reasons, I personally feel I shouldn't leave, and I would think that I am "supposed" to be hurting, too.
Maybe, lordxizor, you are in situation (2), and you are trying different things to fix something that is not in your power to fix, and it is frustrating to see that nothing works. I don't want to presume anything, but maybe she is not happy in life generally, in ways that don't have anything to do with marriage or family issues.
Oh yeah, and a lot of people are filled with ire by the suggestion that expecting sex is reasonable. Surely it's reasonable to an extent. Sometime you just gotta do things you don't wanna do. She wants to go to Cirque de Soleil, so you go. You really don't want to, but it's just what you do. So I sort of think chicks should service their husbands (and vice versa, even though that scenario is more rare), because I believe your job as a spouse is to service your spouse. (That said, I could see how phrasing things so woodenly to your spouse would be a turn-off. I think it should just be implicitly understood on both ends that you do things to make each other happy.)