By "legitimate" he pretty obviously meant "actual."

The right believes that many women claim to have been raped when really they haven't.

Because that way, their parents, husbands, etc., etc., may not object if they get an abortion.

The link I provided makes "sense" of what he was getting at: the medieval belief that a woman can't get pregnant unless she has a, you know, that feeling they write about in Cosmopolitan.

(Remember, I'm just the messenger.)