(NSFW) Why are there so many wives that hate having sex with their husbands?

I know many women (as we sit around and talk) that
just hate having sex with their husbands. I don’t think
its the husbands per se just the fact that they don’t
enjoy sex maybe. They say I am one in a few that do.
I have been married 29 years and we both enjoy our
sex lives. Isn’t that the way it is suppose to be? I just
don’t think I would be happy if my spouse just decided
he didn’t enjoy sex so didn’t want to have it. Any ideas
why this seems sometimes the norm?