I think you are disingenuous here. You very much judge the people who think/feel different. And you certainly judge your niece.
There is no one way to enjoy sex. There is no one way of thinking of dating and relationships. For you - it is to consider yourself gf/bf after having had sex. And thinking that commit and marriage is the ultimate goal of any sexual interaction.
But people are different. And even more so - they may be different in different stages of their lives.
I can see how someone in her 20s may be focused on her studies and career, and not want to be committed to someone. Not being exclusive, does not necessarily means having some sort of wild sexual encounters with multiple people, that you imply or imagine.
Personally - I can enjoy sex in a relationship/with feelings, and casually. Those are different sensations and experiences, but both potentially really good - say like liking sushi, but also pizza.
People are different.
Incidentally - just had a conversation about it with a 50yo friend. She is divorced for a few years. Dating, both for fun and to hopefully meet a new long-term partner.
Seeing a new man. After the first time they had sex - he said he wanted her to be his GF, be exclusive. And she said - no.
She is not ready for commitment and labels so quickly. Doesn’t mean that she is swiping on Tinder, or sleeping with others. Just means she is not yet there to be a GF - that comes with certain expectations.