I can't speak for all women, or most women, or even many women, but I have heard a few women admit that after becoming mothers and/or going through the change, they lost all interest in sex. and it had nothing to do with their partners. (Women tell me the damdest things!)

Also, men are not immune to this decrease; when I was young, I thought about sex all the time. Now, I only think of it when I'm awake.