It's really just another attempt to feminize males, I remember someone on Slashcast citicquing a fic and saying the slash relationship was less about two males and more about two girls with strap-ons. If people want to read a fic with pregnancy as it's theme, there is always het.

But I'm very picky with Slash, I don't think there are many males in HP that fit well with my image of Snape. . . and Snapefic is all I read.