I am pro-choice. It is the parents decision (although typically the mother's) to decide if they are ready to support a baby.
My reasoning? Did the prohibition make people stop drinking? Nope. Will a law making abortions illegal make women stop having them? Nope.
I'm not saying I promote unprotected sex, because that's just dumb (unless you are trying to have a baby). Use protection, get on birth control (which is now included in health care plans as far as I know), don't be stupid about your decisions.
I can't support the no premarital sex rule because I broke it. I do, however, support that abstinence is the only for sure way to not have a baby. That being said, kids are like cats. They're curious and they're likely to experiment. We see it every day - even if parents make a conscious effort to hide their kids from it. Movies, tv shows, magazines, books, billboards, people in the store, internet... it's impossible for people not to know.