So your cheating with this man AND spending time with his kids. Cheating is already so messed up but involving children is another level of terrible. Even if the kids aren't his wife's them being involved and knowing about to would hurt the wife even more.

This guy doesn't really love either of you. You could do so much better than to be with a man that cheats on his pregnant wife emotionally and physically (yes the kissing and fondling is physical and will lead to sex).

This 'relationship' that you think you have with this married man with a different family that is NOT you is built completely on sand. If he'll cheat with you he'll cheat on you. And he's showing you that at your most vulnerable moment (being pregnant with HIS child) he'll still cheat. Tell his wife (she deserves to know) and move on. Don't waste your youth and this cheater.