All of which help you to learn enough about someone to want to enter into a long-term relationship with them.
I talked to 22 people whose casual sex relationships have turned into serious, committed, long-term partnerships (and even marriages).
At the year mark of our hooking up, he told me he loved me and we've been together since (three years now — though on and off and rocky).
I was very surprised when the relationship changed because I started to become distant and gave up, even sleeping with another guy I was interested in.
A week into these more serious feelings I came clean, and shortly after we began dating officially. My current boyfriend had just gotten out of an eight year relationship (married for four years) and we started out just having sex.
This isn't to say that the majority of the people with whom we have casual sex evolve into committed relationships; rather, it generally doesn't happen.
The longest casual sex partnership that turned into a serious relationship was three years. We were best friends, hooked up with each other on and off for about six months, then developed feelings.