Used to look at couples, relatively my age, and would think they must be having intense, wild, sex all the time behind closed doors, until a little later in life realized I was projecting my own stuff onto them. And a lot of people prefer the kind of companionship and daily partnership.
There's no doubt that domestication/domestic slows down the sex, and there's also the idea that once you become a "wife" "mother" or a "girl friend", both men and women have different ideas about what sex looks like with their wife, rather than their previous lover or girlfriend.
part of this is sex has been such a long taboo subject, both in the marriage and outside, kind of like who makes what salary in a workplace. People feel like it's super weird they aren't having sex, and so they don't talk about it with friends, who could confirm that they too were in a similar place.