Other: Happen to be gay, causing the question to instead warp into one more about stances on absolute/partial/non monogamy. Fun.
That said I generally tend to believe that were I involved with women I probably wouldn't stick around after being cheated on, baby or no. Ultimately it's her responsibility since she should've been, you know, one the pill/having him wear condoms, if she gave a shit about avoiding the situation. If he's real human garbage then I'd consider finding a (legal, though maybe at bit immoral) way of getting him to go away, encouraging her to seek comfort among her family as I want little more to do with her.
I'd be more tempted to stay if it was a boy but still would probably still end up leaving.