Please, ... uhm ... is it a sin to continue having sex with my girlfriend, who already has a baby with me?
Let me ask you a question as well: Would it be all right to continue stealing from your neighbor because you stole from him before? I would hope the answer would be, "No!" Just because a sin was committed in the past, it doesn't cease to be a sin.
In the same way, having sex with anyone when you are not married is a sin. "Marriage is to be held in honor among all, and the marriage bed is to be undefiled; for fornicators and adulterers God will judge" (Hebrews 13:4). The fact that a child was conceived by your past sins doesn't change the fact that having sex outside of marriage is a sin.
Thus, an important question for you: Clearly you plan to continue this relationship, so why are you not getting married? Then there would be no question whether having sex with her would be wrong when she is your wife.
Wow! Thanks! Thanks a lot for this information. I'm so very grateful.