Where is this scene you're referring to?
It's a defeat scene. The only criteria it says is that you cannot have an already-active anal pregnancy.
Edit: it seems that even normal pregnancy seems to stop it and that's what happened, I got it to work by going back with no pregnancy, and it came up as normal. Oddly, it is blocked by vaginal pregnancy, but it does not block vaginal pregnancy... maybe something to do with the status effects?