I've heard that unwanted pregnancies are more frequent in the "wait till married" crowd. That is because they are unlikely to have condoms with them when their emotions overwhelm them. They don't account for their changed decision making process when aroused. (Source: Dan Ariely's Irrationality MOOC).
That's what I've seen from personal experience. Thinking along the lines of: Birth control's for sluts, and I'm not like that, so I don't need that, this is special, etc.
