If the girlfriend is married, her husband might be her primary partner. She and the guy you like might have a relationship where they do not consider each other to be "primaries." The guy might, in fact, be looking for a primary partner of his own.
You can't know without asking him and talking to him about it, of course.
You can also ask him how he would label/describe his relationship with her, since he does not seem to use the label "girlfriend."
I can relate to that--I am involved with someone, and we do not feel the labels "boyfriend" and "girlfriend" describe our relationship. We are not each other's "primary." (He's looking for a primary partner / "girlfriend", and I don't want a primary partner).
But don't assume that my situation describes your guy's...just ask him about it!
Single, straight, female, solo, non-monogamous.