I think I'm homophobic?! {complete}

This straight guy, who we'll call Bucky

         So there's this straight guy, who we'll call Bucky, who's been roommates with this other guy, Steve, for a solid amount of time. And you see, when Steve told Bucky he was bi, he was absolutely fine with it. But then when Steve started to bring guys home, Bucky started feeling annoyed, resentful- disgusted.
        So Bucky turned to the internet for help. What better way to figure out his seemingly homophobic tendencies than to ask strangers on Reddit for advice.
He flopped onto his bed and grabbed his laptop. His fingers hovered over the keys, waiting. After a moment of contemplation, he started typing.
           First things first, let me say that I've never thought of myself as being discriminatory before. I had a gay friend in high school and we made it through some tough times together, I never felt weird about him dating a guy. So all of this is coming out of nowhere.
        Steve had been Bucky's roommate for over a year now, and Buck pretty much knew upfront about him not being straight. The two clicked instantly and quickly became best friends. They joked around, played dumb pranks on each other, hung out on weekends, went out for greasy burgers after bad hangover; sometimes Steve would pretend to flirt with Bucky and Bucky would pretend to flirt back.
          But Bucky was straight and Steve knew that, and Bucky didn't feel threatened by Steve flirting, which surprised Steve. Most straight guys' masculinity was to fragile for anything like that in his experience.
          The problems started because of this: Steve brings guys home sometimes. At the start I thought I was okay with it, since it's really not my business who he sleeps with. He's usually discreet enough about it that I don't see/hear anything I wouldn't want to see/hear from anyone else, but for some reason I've started feeling weird if I even see him with other guys.
           Bucky thought back to the day that made him realize there was something wrong. It had been a fucking exhausting day and he was ready to throw himself into his bed and curl up under the covers when he finally got back to the apartment.
       I came home and saw him and some guy making out on the couch. I don't know how to describe what it was like to see that, except that for a moment I felt so bad I thought I was going to throw up. Steve was embarrassed (he didn't think I'd be back for a while), but I told him it was okay since I was embarrassed too.
       If he was being honest with himself, Bucky was still embarrassed. He spent the rest of the night hiding in his room, even after the guy, Tony or whatever his name was, left.
         I felt bad for being as disgusted as I was, since there's NO good reason for me to have a reaction like that. I thought maybe they just caught me by surprise and that's why I reacted so strongly, but it turned out it wasn't a one-time thing. After that, every time he has a guy over (not that often, but every once in a while) I just start feeling like shit and wishing that guy would leave, and I can't stop thinking about what these guys might have done to him even though I don't want to imagine that. It makes me really uncomfortable and grossed out. And these are just guys he fools around with, I don't know what I'd do if he ends up getting an actual boyfriend.
         'God that would fucking suck.' Bucky thought bitterly as he closed up his laptop. 'Okay, but why?' Bucky asked himself. He spent the next hour contemplating the question, trying to psychoanalyze himself.  Eventually, he drifted off into a peaceful sleep. When he woke up the next morning, he knew he had had an amazing dream, one of the ones where you don't want to ever leave, but the memory of the dream was lost as he rubbed the sleep out of his eyes and prepared for another days work. He tried to picture the images that had been playing in his mind not 10 minutes earlier while brushing his teeth, but nothing came up. The only pieces he could remember was laughter and piercing blue eyes with small flakes of green that crinkled in the corners with every smile.