Maybe a better example is how we all drive cars. There's a question on the DQT that reminds us that "Driving is one of the most dangerous things people do", yet we rarely think of the responsibility entrusted to us when we take the wheel. The lives of your passengers, people outside your car, and yourself are in your hands, and even if you don't "mean" to have an accident, when you do you still have to face the ramifications of your accident.
There is no greater responsibility than the creation of a human life. Whether or not you intended to create that life is irrelevant. Your actions, in committing the act of sex, CAN result in the creation of life - and if that creation of life happens, then it is a responsibility that the parents MUST bear.
Abortion gives us two broad options. We can consider the impact on the parents, or we can consider the impact on the unborn child(ren). Is the termination of human life (no matter how fragile) more valuable than the convenience for the parents? If our parents were in hospital, would we just ask the hospital to pull the plug so we could get at their estate? Because that is, for all intents and purposes, exactly what a mother and father having an abortion is doing.