Sex is always the first thing to go once any kind of resentment/contempt creeps in. That can be resentment for having to do and think about everything in and out of the household ie, washing, cooking, cleaning, lunches, sorting Drs/dentists/opticians, reminding partners of their own families birthdays, buying the cards, gifts, carrying the weight of Christmas and other special seasons, acting as emotional support creatures for everyone in the house, buying new clothes, buying new clothes, caring for elderly parents, nursing sick family members. On top of this, many women also work of course.
Then there is the fact that many men see us as 2 dimensional creatures rather than real life, flesh and blood humans with our own interests, opinions, feelings and can't see past the idea that we only exist for others. Women generally want to be appreciated for who they are rather than simply someone who exists just to serve.
Ogling other women- this is a big one for me personally. I'm not an idiot, us women know that men are visually stimulated but if I see you constantly checking out other women, whether that be in the street or on a screen (either on Social Media or porn) then you are telling me two things 1) you have no self control which I don't respect and 2) you don't respect me enough to not make it obvious. This will make me lose both respect and attraction for you and sex will leave the chat almost immediately.
Other reasons for resentment are if you broke our bond/commitment in one way or another such as lying- women don't generally want to be intimate with a man we don't trust as he no longer feels safe and quickly starts to feel like a threat. Nobody in their right mind wants to be vulnerable with somebody who makes us feel unsafe.
We don't fancy you anymore- this doesn't happen in a vacuum, we tend to start off finding you very attractive but any/all of the above are death by a thousand cuts and the end result will be we simply do not fancy you anymore. Any attempts to connect by you are likely not to work as the kinds of attempts men tend to make are further off putting such as 1) dry humping us or grabbing/petting/groping us while we go about our day 2)Whining/sulking/demanding/getting angry or aggressive at the lack of sex 3) buying things to help aid sex life, because all these things go back to my original point that we end up feeling like a 2 dimensional creature that you want something from.
The key is usually communication (though if we are too far gone, even this won't help nothing will) to try and get to the root of the problem. If you handle this badly by claiming we are nagging/complaining, if you get defensive or angry, basically if you display no ability to listen and understand how we got here then we will move even further away from you emotionally and we will want sex even less. We may attempt a few more conversations on the topic but if there are repeated failures due to your inability to listen/accept/empathize with us then we will eventually give up completely.
I hope this helps.