Look, you’re being hardheaded. I think you know the answer to this question as well as any other human being, and this thread is purely masturbatory/condescending, but I’ll indulge you as if you were not a member of the human race and therefore had no idea of how our crazy little minds worked.
It DOESN’T change when they enter a relationship. They still think NSA sex with multiple partners is ok. They just don’t want to do it anymore.
And before you go, “But whyyyyyyyyyyyyyyyyyyyy???”, take a minute to think about it. You want different things from different people. You want your boss to be professional and courteous, your mother to be kind, loving, and supportive, your friends to be fun and good-natured. You want some random hot guy at the bar to be good in bed and un-psychotic. You want a potential emotional partner to be that (ideally) and charming, intelligent, witty, devoted, and any number of other adjectives. When you’re single, you’re a whole lot more likely to meet someone who is just attractive and willing than all of those other things, and if it’s at a time when you feel that particular stirring in your loins, you go “what the hell, why not” and have some fun. When you do meet someone who’s emotional-partner material, if you’re at a time in your life where you have time and emotional energy to invest, you do so, after which you may form a bond that is both sexually AND emotionally fulfilling, so you don’t have the need or desire to have casual sex anymore.
It’s like with music. I can turn on the radio and hear any number of catchy, poppy little tunes that make my foot tap, and sometimes, that’s just what I do. But sometimes, I feel instead like hearing a symphony, something soul-stirring, and in that moment, I wouldn’t change the station looking for Britney Spears. Of course, that’s not a perfect analogy, but the point is that sometimes in our lives we are looking for fun, and sometimes we are looking for something deeper. It’s ok to enjoy both, and if/why we eventually change our minds is only explicable on an individual basis, because there are so many factors that contribute.
I think what you’re really having a problem with is the idea that sex is not always sacred to all people, and honestly, if it is ALWAYS sacred to you, no amount of explaining is going to make you understand why it’s not to others or make you feel any less repulsed by those people. The fact is, sometimes, it’s just two bodies rubbing together in a way that feels good, and sometimes, depending on the partner and the mindset, it’s that and much more. You can sit on your high horse all day saying “but if the latter is superior, why would you EVER want the former???”, but the truth is, relationships require a lot of work, a lot of time, and a lot of energy, and some people would just like to have a good physical feeling now and then without the added baggage of having a partner whose every need you must be attuned to at all times.