"The hooking-up phenomena has been traced back to the 1960s and the 1970s, when male and female students were thrown together in apartment-style dormitories..."- Brenda Wilson, in NPR's recent article on the hook up culture.
I'd like to give an honorable mention to the "hook-up buddy" before launching on ambiguity. The hook-up buddy is a friend, acquaintance, ex-lover, or stranger with whom one enjoys a no-strings-attached sexual encounter. Think... Sex and the City. In fact, read some wikipedia on it, and discover that this show "expressed true adult comedy and sex in an up-front way." Oh so now that you're an adult, you're mature enough to enjoy casual sex? As opposed to before, when your emotions were hard to control?
My problem with hook ups is this: in hooking up, you engage with a person on an intimate level that does not reflect any intention of commitment. You're expressing something with your body that you deliberately withhold with your intentions. Hook ups aren't about meeting persons. They are about meeting bodies. They are a mutual agreement to have a good time. Can anyone say dualism?
If you're a woman, you're also shooting your emotional stability in the foot. A mere twenty second hug releases oxytocin, a bonding & trust hormone, in a woman's body. The morning after a hookup, a woman is chemically changed- she has been emotionally glued to her hook up buddy in some way. Sure, she can convince herself that it's no big deal. Hookups are transient happiness, however, and probably aren't worth the lasting emotional baggage.
...And a brief word of thanks to co-education. Co-ed dorms: making physical intimacy without corresponding commitment an easy possibility, every day.