Re: but ew, cooties!
It wouldn't even have had to have been specifically when Peter and MJ began bumping uglies. Could have been before, could have been after. Could have been Peter just worried about his own health--I could swear that the "radioactive blood" (cited in the lyrics to the original cartoon series) was brought up as a plot device during the Romita Sr. years. I'm suggesting that, logically, at some point, Peter either would have brought it up in a conversation with Reed Richards or someone else would have brought it up with him--if not Richards (because he's a huge nerd) or Johnny Storm (because he's Johnny Storm) then Ben Grimm, especially since Ben would have his own radiation-created problems with sex (but we don't need to talk about Alicia and the rock-polisher right now).