I think in some ways you're overthrowing this OP and your previous lifestyle and being swingers etc is making you feel like you should be more open to something that is in fact really simple - a deep betrayal.
You've said yourself that swinging was all agreed upon, talked about, boundaries were put in place etc.
He then, without even telling you how he was feeling or asking if you'd reconsider, had an affair behind your back. Doesn't matter if its a man, a woman, or what exact sexual acts he did, he betrayed your trust. That is absolutely massive.
Just because you've agreed on something in the past, doesn't mean it's always ok...for example someone asks to borrow my car, and I say yes ok, just until next month when things are changing and I need it every day...doesn't mean they can just take it whenever. It turns into stealing. Just because I've had sex with someone willingly in the past doesn't mean they can just have sex whenever they like. That's rape. And just because you've been swingers in the past doesn't mean that you have to be ok with him having an affair.
Instead of getting angry, distraught, chucking him out, giving yourself chance to process your feelings, working out if the relationship is worth fighting for, letting his behaviour show you how sorry he is etc, all the normal stuff, you seem to be trying to put pressure on yourself to work out ways of accepting it. Questioning whether you're homophobic because you don't like the idea of your husband shagging another man in secret, when you've expressly told him you never want this to happen under any circumstances, is madness. Trying to work out why you feel so bad...when any human in any kind of relationship would feel bad in this situation - because he has treated you like shit. And he is continuing to treat you like shit.
You don't like him sending you pics of men, because it shows a. He is not even fucking sorry, b. He is not trying to make it up to you and build the trust back up again and c. these two things mean he is probably going to do it again, because frankly he doesn't seem to even care about how much he has hurt you, because he is justifying it in his head as 'expressing himself', and your hurt is an inconvenience to his new true self. Most people are mortified when they hurt someone they love, they don't rub it in their faces and then whinge when they're upset.
I think his actions since this, completely stink, and it's making you second guess whether you're actually to blame, which is almost a form of gaslighting.
Bottom line, you had a line in the sand, he passed straight through it, and doesn't even seem to care, and its normal to feel gutted, even more so given his behaviour since then. I think you need to stop the introspection, accept your feelings are normal, and work out if you still want to be married to someone who is acting like him