Because most women dont think the same as men and due to hormone changes (frequently) with women a lot are just not really interested in sex and of course the act of sex is not what works for most women but it's the emotional buildup to it, very different than guys.
A lot of women turn off the sex after marriage. I have several friends who have complained about that over the years after marriage. Its like they will pretend to be all into it when they are dating and once married it just turns off. Thus the old saying, do you know what food group kills the sex drive in women? Wedding cake.
