Casual sex pretty much defines my (non-masturbatory) sex life.
At one point in my life (and I am uncertain if it was inspired by someone else or if it came from my own deliberations), I started thinking of sex not as a sacred/profane activity but as yet another form of communication. Sex is but another, more intense way for us to interact with each other.
While I have let myself explore some of the emotions that come with affection, crushes, and romantic-sexual love, I don't aim to get involved with anyone in anything more "serious" than a friendship.
Combining these two factors, it was easy for me to define and seek out friends who accept sex as part of our means of communication.
Part of this includes meeting potential friends and maybe hooking up on our first meeting together. Sometimes, the first meeting ends up being the only one and, in effect, "just a hook-up."
I've also gone for straight up hook-ups; even then, my curiosity about people is active and I want to know more about the guy I'm having sex with (even if we don't have known intention of meeting each other again); I understand this added level of interest doesn't work for all guys.
I should add, for posterity's sake, that I (unfortunately) only infrequently meet with guys for sex (largely due to logistics and my unwillingness to leave the comfort of my home; once I have my own place, my meetings will probably become more frequent).
My sexual fantasies are far more risque than I am though and include much more "casual" sex.