"If it's a choice, you're pregnant" is not logically equivalent to the bumper sticker, but "If you're pregnant, it's a choice" would be, as per contrapositive inference. Which makes it sound kind of like a pro-choice stance. Which is what the original sounded like to me as well- if you don't have the choice, then you aren't pregnant. If you are pregnant, you have a choice.

I dunno. It's still confusing.