Reply #103: the main question I see behind that logic is:

[fnottr]

Does the act of having sex imply a consent to become pregnant? Some groups (the Catholic Church comes to mind) would say yes. However, I would argue that humans ability to divorce the act of intercourse from procreation is one of the things that separates us from lower animals. It is (to be a bit cliche) one of the things that makes us human.

Pregnancy can occur from sex in the same way that becoming injured in a wreck can occur from driving/riding in a car. Even though both might be undesired outcomes, everyone knows that either can happen.