Maybe because a lot of people to tend move too fast in relationships. It happens all too often that by the time you start to really know someone you've already had sex, moved in, etc. and it's too complicated to just walk away at that point. I suppose it could be an 'out', especially at a young age when you really don't know what you want and your interests are changing as often as your underwear.
“Be who you are and say what you feel because those who mind don't matter and those who matter don't mind.” ~ Dr. Seuss