Yep, and of course you can go on with your life and have kids, etc. I mean, let's see -- if your boyfriend had been married before, and was divorced, would that taint him somehow? Or if he was widowed? Or just had previous relationships (which I suspect is already the case, yes?), would that somehow make him unworthy?
So why are your potential future children somehow now unworthy of being born because there's an older child?
Hey, stuff happens. The guy and you were split, yes? That means that he (and you, BTW) doesn't have to sit alone at home, crying and pining away. It means that, sometimes, people have sex. And sometimes that sex leads to kids. In this case, it did. Sorry, but that's the case.
Personally, I'd consider myself damned lucky to be with a guy who was that open and willing and accepting of his responsibilities (assuming the child is his) for his actions. This is actually, in many ways, a good thing, believe it or not.