There is some sense to the 'in case of rape' argument, tied in with the Castle Doctrine line of thought. If someone is trespassing on your property and threatening you, Castle Doctrine says that homicide is justifiable. However, it doesn't apply if you invite someone in, as it isn't then trespass. An unwanted pregnancy can be considered analogous to this, and the (normally pro-life) line is that CD doesn't apply after consensual sex because that's 'inviting the pregnancy' - however it does apply in cases of rape.
There are all sorts of potential holes in this (both from a pro-life and a pro-choice POV), but that's the jist of it.