He also just might not be ready to date. Like what other people say, he could be afraid of rejection but I'm not so sure about that. He might be afraid of losing you as a friend. He's afraid he's second place to your new girl and he's not ready to date them. A friend of mine has this problem with a couple of friends of hers. Once she and her BF started going out the couple of guy friends of her bf's (let's call him Todd) would constantly put her down and just continuously harrass him for dating her and not spending any time with them. However, when one of them found a girl of their own, they'd spend no time with Todd. They acted the way they did towards Todd because they felt rejected by my him. They wanted more time with him. Maybe your friend is afraid of the same thing. He's just not as outgoing as Todd's friends are.
If he's gay he'll discuss that when he's good and ready but for now, there's no reason to assume that. It's good to talk to him though about the way he seems so withdrawn around girls and around you when you are talking about girls or even the girl you are with. Ask him if there is somethign you could do to make him feel more comfortable. If the answer is for you to stay away from your gf, then maybe you'll have to make a compromise to see him alone. but other than that, if it's just talk of girls or being around girls, then maybe he does have some sort of social anxiety about girls. Something from his past maybe? You'll never know unless you talk to him.
