Stay or leave

I just found out my boyfriend is having a baby with another woman. The time that she became pregnant we was on a break but he didn't tell me he gotten someone pregnant until recently when he got the sono pic showing the sex of the baby. I confuse on whether I should stay because technically we wasn't together at the time or leave because the possible drama that's going to come with the mother of his child who his childish and him being in a relationship with me. What would you guys do?