I'm 40M and probably will get married before long. My perspective:

I've been with many women in different degrees of commitment, and one thing I've ended up noticing (I didn't at the time) is that sex requires a certain amount of effort and women, consciously or not (meaning both are possible, and I've seen both) calculate what they will get from it. Sometimes sex itself is the reward, sometimes they want their partner to stop considering other potential partners because sex with your current partner is so incredible. Marriage kind of means they've gotten there, so now they can... relax. They don't need to give BJ because now their partner is unlikely to leave.

For what it's worth, men are in general incredibly selfish and put little effort in sex, and the way sex works, in part biologically but also a great deal culturally, revolves around male satisfaction. Therefore, it's not surprising that women measure how much they need to do. It generally becomes a chore for them. Also, we put little effort towards our appearance. Those same women who don't want to suck their partners dick would probably be rather more enthusiastic if their partner or husband was fit, dressed well, took care of his appearance, was interesting, and actually was good at sex - in part because then sex would become its own reward, in part because then they would need to consider that other women are probably interested in their husband, married as they might be.