Plan B is a pill that you take after somebody cums in you to prevent a pregnancy. Since plan A is to use protection, plan B is used when, for some reason, you could not have used protection.
I'm aware. What I was joking about was the ambiguous wording that could be taken to mean that someone came in someone in order to prevent pregnancy. Potentially analogous to turning on a lamp to prevent light 😛
Reminds me of this passage from Hitchhiker's Guide to the Galaxy:
The renewed shock had nearly made him spill his drink. He drained it quickly before anything serious happened to it. He then had another quick one to follow the first one down and check that it was all right.