It doesn't really change things does it? He could be in a miserable marriage and still got her pregnant. Only takes one time ya know? It's really not your concern she's pregnant that's his thing to deal with. Sorry I really don't get the difference lol. It's not like you're any worse of a person just because she's pregnant. I'm obviously not faulting your decision, just trying to understand the rationality that cheating is only wrong when the person's SO is pregnant.
Maybe I just don't understand affairs because I always assumed it's about the connection two people share. Sure if you were planning on building a life together it complicates matters but if it's just a physical type connection it's irrelevant.