ANDY'S ANSWER: Like almost anything else, when something controls you instead of you controlling it, it can become an addiction. Sex feels good, and for that reason some people don't use their brains when making a decision about the right time to have it.
There are lots of things to consider before having a sexual relationship that go beyond the actual moment of passion. One of the reasons that there are so many accidental pregnancies and sexual infections going around is because the individuals were not mature enough to think about the responsibility that goes along with having sex.