Since women will start becoming less attractive to men starting sometime in their mid to late 20s, what's the point of having a relationship with a man once she becomes middle aged.

He will be put off by her aging and will desire young women. This is of course amplified by the Coolidge Effect.

It's highly likely that he will either cheat on her with a young mistress, or if he can't do that he will cheat on her with similar aged woman, or pay for sex with a young woman.

If he doesn't cheat, he will probably want to and will quietly or openly resent her for aging.

There is a very high chance that a woman will spend years of her irreplaceable youth with a man (remember that since the average age of marriage is 27, which means couples begin in a woman's early to mid 20s) only to end up with a husband who cheats and/or resents her.

Of course some men will not resent their wives for the inevitable passage of time, but they are a small minority.

It would be better for women to dedicate their time to bettering their own lives and their communities or dedicating themselves to a cause they are passionate about. If they want children, women can seek sperm donors. If they are bisexual perhaps they can get a gf/wife. Masturbation and sex toys allow for sexual satisfaction for straight women. And if they really want to, most women can easily get casual sex.