Yes, this is a guy thing. Sometimes we don't orgasm no matter how good the sex was. Same thing happens to them.
My boyfriend never ejaculates during sex, he has to finish himself off after I'm done. If I had to wait for him to finish sex would've lasted 4 hours and I'd feel beaten afterward. I also felt he doesn't like me and that's the problem but then I realized it's not. He says he would have to thrust really fast in order to finish which is impossible during sex (he'd die of exhaustion lol me too) and it's much easier to do it by hand. And this is every time we have sex I'm talking about. Not once or twice. But I know he enjoys sex as he always initiates it and we had no problem getting pregnant.
Also some guys (many of them actually) don't like opening up about thoughts and feelings.
Just leave it be or he'll feel like you're smothering him.
He might want to go easy on masturbation so often if you guys have regular sex.
"All he said was it wasn't me, but of course I felt like it was." Guess what, it probably wasn't you. (At least the first time.) You should have believed him and let the subject go. Does he anxiously quiz you after every sexual encounter, to know if you had an orgasm? If he did, wouldn't you feel kind of pressured? What would it do for your sexual performance if you feel you had darn well better orgasm "or else" next time you had sex? Sexual anxiety, guilt, shame, and even just being tired, distracted or bored, can all affect the ability to orgasm.
You're saying that either things are all right or --- he's LYING. That polarized of a response to the situation doesn't help. He might have known what happened but was afraid you would take it as a criticism of yourself (even if it wasn't), and he was trying to get you to drop the subject. And then you quizzed him the next time, and say he might be a liar, and the weight of the world seems to be on it. If you want to drive him away, keep up this approach.
So you can try to suss out what might have occurred without harping on him, let's consider some possibilities.
1. The first time, he didn't finish for whatever reason (tired, recently masturbated, mind wasn't on it, things going poorly at work, got distracted) and you reacted so personally that he never wanted to go through *that* again. Then he got anxious the next time and couldn't finish (performance anxiety in sex is a genuine thing), but didn't want to tell you for fear you would act like it wounded your self-esteem and demand to know if he still cares about you, etc. Not feeling like explaining performance anxiety could indeed be a "guy thing."
2. He's been masturbating a lot and doesn't want to tell you, because he's embarrassed, or because he's getting kind of addicted to porn and doesn't want you to know, or because he thinks you will be wounded in your self esteem (as in #1 above). That would be a guy thing, to not want to trigger your "of course I thought it was me" reaction. We get letters a lot from women who think their boyfriend is cheating on them if he watches porn. If you've said something like that to him, he isn't going to volunteer the information. A lot of guys do watch porn and jerk off, even when they are happy in their real-life relationships.
3. He's tired of the same old bedroom, the same old sheets, the same old moves, the same old routine, and sometimes it fails to inspire him to orgasm. Not wanting to fess up to sexual boredom would be a guy thing if he thinks he's going to get the third degree about it.
4. He's gay and closeted and genuinely wants to have a relationship with you, but sometimes it's just not quite there for the sex. (This one, mercifully, isn't as common as it used to be.) That would be a "guy thing" for sure, and a complicated one.
5. He's seeing someone else, but is still trying to keep things going with you, either because he sees the other thing as very temporary and you as long-term, or because he hasn't worked out what he wants from it or how to tell you.
I'd suggest it's either performance anxiety, in which case you bugging him about it will just make things worse, or he's having orgasms elsewhere. If he's having orgasms elsewhere, it could be by himself (and if you would challenge him about why he does, he might well decide it's a guy thing and not want to tell you about it). It could be with someone else. If it's with someone else and he's not breaking off with you, it could be because he's trying to decide what he wants.
So, my suggestion is to low-key it and stop demanding to know, and stop accuse him of being a liar. Time will unwind whether your worst fears are realized or this was just a temporary inability to orgasm that has been blown up into a bigger deal than it ever should have been.