It could be a bit of both. I mean if one of my guy friends had a girlfriend and didn't tell me about it, I'd be a little put off. I mean I'm his friend so it's logical that I should know something about it. But I feel like like I'd be more put off if this friend was special to me regardless of gender.
Like one of my guy friends who I love like a brother has a huge crush on this other girl. I get a little jealous when he talks about her, talks to her, or when he makes some new advancements with her. Even during the time I had a boyfriend I would still feel a bit jealous. But I don't want to date him nor do I really see him in that sort of way. So I feel like it's pretty normal and you shouldn't worry about feeling that way. Only if you start to realize that it's more on the spectrum of you liking him should you be worried.
Instead, you should feel more worried about him not telling you. You are his friend after all. I think that you have a right to at least know about it. If you tell him this and he thinks that you're overreacting, then tell him that dating someone is a pretty big deal and as friends you should be able to know something about it. It's part of being friends: You tell each other about big life events and don't keep those big life events a secret from each other (unless those events could potentially be hurtful and in this situation I don't think that applies). You don't need to know the nitty-gritty details about his new girlfriend or the relationship, but I feel like you should have at least known something about the existence of their relationship.
