How have you felt about your relationship lately? Is there any distance between you that didn’t used to be there? Have you felt like there’s anything missing in his presence with you? How have things been sexually? Does he want more than you want? Do you want more than he wants? How has he been, otherwise, in the relationship? Do you still feel close to him? Are you connected with him?
I would believe him when he says he’s never met anyone in person. Online sex can be very intense. It can be a way to blow off steam and to feel like you are still attractive, as well as to get your rocks off. It can feel like it’s same, because everything that happens is in your head. No actual touching of body parts or exchanges of fluids. Many guys would think “what’s the big deal?” It’s just porn and whacking off, except in his case, he’s showing off to guys. He may not even be interested in any physical relationship with a man. Rather, it could just be the sense of being powerful and attractive. Some guys like showing off their privates, especially if they haven’t felt all that good about them before.
Sometimes people can do this even in good relationships. It’s not about infidelity, although I’m sure you interpret it that way. It’s about finding something you feel is missing from your life—excitement; meeting new people; feeling like you’ve still got it.
As the @Psychedelic_Zebra said, the fact that you went and searched his history is a sign that there are deeper problems in your relationship. I would ask you to think about how long you haven’t trusted him? How long has it been since you’ve been wondering what’s going on? Have you felt these things but been afraid to talk to him?
And on the other side, has he been asking for things and you haven’t wanted to give them? Does it seem like he’s frustrated with you? Have there been a lot of fights and sullen silences lately?
Couples counseling, of course. It helps you talk. If he doesn’t want to go, it may be because he’s afraid you’ll beat him up. He already is ashamed of what he did, and he isn’t interested in giving you more opportunities to make him feel worse.
A good counselor will help you both express your anger without beating each other up, or perhaps just help you get beyond the anger to problem solving. I have to ask if there is a problem. Have you been missing anything from him? This doesn’t have to be a blow to your ego. It doesn’t have to mean you should feel bad or inadequate. You don’t have to be threatened. For him, it may just be a way of whacking off. He may have no relationship at all with anyone. All they do is display themselves to each other.
I think this is a shocking thing that you can’t wrap your mind around. I would ask you to try to let time go by until you can see this without the haze of hot anger. It may not be whatever you are imagining it is.