Ok to the question "why" we cannot give you an appropriate answer, only he does. It seems as if you came here before even asking him what is the problem and what's going on.
About the strap on part, the fact that you had that type of sex is in NO WAY indicative that he is gay. A lot of straight men like anal stimulation and some of them even partake in pegging. It is not so much about experiencing sex with another male but about having the woman in dominant position. Google a little bit about that and you will have male perspective on pegging and if it made them gay or not.
But if there is anything else going on that makes you believe he might be gay you should've told us, it is rally hard to deduce from the info you gave us here.
Again talk to him, see what's wrong and then we can try and help you from there.
So, have you had a heart to heart with him about this??
I just would like to say that some men like to take a passive role. I enjoy being a sub sometime. You might try starting it tell him to do what you want be forceful see if this helps
Maybe as a middle ground between challenging him directly and just going along with a feeling of heartbreak and mistrust, you could try suggesting various switch-it-up sexual roles and just see what he goes for. Maybe he's liking being submissive and that's all it is, or maybe he is a closeted gay. But if you can't find a way to open the conversation, either by directly asking him or by subtly hinting or testing to see how he reacts, you may as well not be married. Honesty, however you get there, is more important in a marriage than sex.
The sex part isnt tearing apart your marriage. The lack of communication seems to be the factor here.
I seriously hate the way that a man doing anything other than some flavorless nonsense that everyone, at least, publicly, finds acceptable, somehow makes him gay. A woman showers with another woman and can even soap her friends back without anyone thinking anything other than they are good friends; however, if a man even looks to see who walked into the showers while he washed his hair, somehow, instantly, his masculinity (itself, absurd, no one thinks the aforementioned two women are any less feminine) is suddenly in question. I am, generally, very dominant in bed and my partners have been happy with the dynamic as well as the sexual components. That being said, just as femdoms make their male service them, every once in a while, I need a really good seeing to! When the feldoe first came out, I was crazy horny! Disappointing, if you have to worry about it then you are not focused on my pleasure or the mindf__k you are getting in return.
The guy might be getting off on something that women that have not put on a strap on and serviced a man can't know: f---king is hard work and getting pleasure out of the workout is harder than people think. Think about it, for once in his life, he can just relax. All of the sudden, all the irrational things that stress a man out in bed are irrelevant and multiple orgasms are a real possibility... maybe... even, the holy grail, the hands free orgasm! Being gay is not about having sex with a man, being gay is about falling in love with another man and growing old together. Being gay is when being told the cancer is benign causes both of you to hug and cry tears of relief --as any couple would. Now, relax, buy a lube shooter, a spare parts harness, the right dildo --headless is better for beginners-- his twin, and work on some conditioning for the workout; enjoy!
Having said that, I would like to echo AnnieBrooke and DominoSarah and say that communication is key. I would add that thinking in terms of dirty or shame or disapproval will only serve to end what you have. Go into this looking forward to another world of adventure and play and not thinking about what our cultural indoctrination (idiocy dies a hard death eh?) tells you. Just imagine, being able to say out loud things like "OMG! check out that c--k!" knowing that he will turn around and say "oh yeah, that is a gorgeous c--k!" without fear (he might not like THAT particular one but so what?!) and, if you are confident enough, "baby, check out her ass!" Remember, if he likes that one, it does not mean he likes yours any less any more than you like the c--k any less than his. Masturbation? his hand does not judge him for what made his c--k hard.