Well before even tackling that question, there is the underlying question you have to ask yourself: Is it a dealbreaker to you that he has a gf (or a bf) on the side? Or put another way, is monogamy a major priority?
If you guys are not serious/exclusive or are poly/non-monogamous, then you would have to discuss how a hypothetical gf would affect your relationship.
If you haven't read any of Dan Savage's "Savage Love" columns, this is something he has responded to in many different permutations, so definitely check out his column archives on The Stranger's website, and even read some of the comments left by readers. It may help you sort out your own feelings on the subject, and consider all sides of the issue, good and bad.