How do I get over my boyfriend fooling around with his roommate?

I've been dating by boyfriend [gay] for almost three years. I always joke with him about how close him and his friends are. I would say things like, "You could totally get with him," and, "We should have a threesome." It was said jokingly, so I was not worried. He became roommates with one of his straight friends. They are close and kinda play flirt with one another. Well, My boyfriend just celebrated his 21st birthday and got drunk (maybe just buzzed) for the first time even though he told me he had to work and wouldn't be drinking. I was disappointing because I wanted to be there for his first time being drunk. He said when his friends brought him back home, they laid him in bed and he stayed there until they left. He then got up and took his pants off. So he was in bed in his underwear when his roommate, also drunk, came in naked and with an erection. My boyfriend said he doesn't know what he was thinking, but he moved over to make room for him. His roommate climbed into his bed and got under the covers. They cuddled and began grinding. My boyfriend said he got curious and reached around and started jacking him off. Also, at one point, he decided to squeeze his butt while he was humping him. He said he never took off his underwear. Then they just fell asleep like that. Then my boyfriend woke up maybe an hour later and decided to roll over and start sucking off his roommate. He said that lasted about 2 minutes before his roommate said, "You know I'm not gonna cum, right?" So my boyfriend stopped, jacked him off for a minute or so more and turned back around. They cuddled and fell back to sleep for another hour or two. Then my boyfriend woke up and told him he needed to go to his bed. That's how the story ends. So here's my question: How upset should I be about this? I don't know how to feel! I'm devastated, but at the same time I was always alluding to it happening - I put the idea out there into the universe. We used to fantasize that I was his roommate while we fucked. We had sex on his bed once. I mean, his roommate is cute and has a really nice butt, but I didn't expect anything to ever really happen, he's straight! Do I really have the right to be this angry? It took him two days to tell me the complete story. At first he was going to lie to me and say he just saw his penis, but then he said that made him feel bad. He was scared to tell me, so at first he said it was only a hand-job. He saw how much that upset me, so he didn't tell me the whole story until the next day after I cooled down. So for about a week I was upset. We cried together and talked. We're working through this. I just hate not being able to trust him. I mean, its his roomate! They live together. I haven't said anything to the guy, either, so he talks to me like everything is ok, not knowing that I know. It infuriates me, but he's not a bad guy. I don't want to hate him, but what am I supposed to feel?! They are still Snapchatting and texting each other like nothing happened. They were drunk, He didn't hide it from me, and I don't want to be mad. I just want to feel the same way I felt before all this happened. Is that possible to do when I fear it will happen again given the right circumstances? I also feel like I'm not getting the whole story. The bars close at 2, and they are only about 20 min from his house. He says it happened randomly and he didn't expect him to come into his room, but I saw where they had a conversation for 11 minutes at 2:30 a.m. (He doesn't remember) and a Snapchat at 3 a.m. (he doesn't remember). He says he doesn't remember how long he was in bed before his roommate came in. I feel like there's more to the story even though he swears there isn't. There's a possibility that I feel this way because he withheld information from the beginning and considered lying to me. I don't know. I just want to move on, I feel like we are making progress. Its just a nagging feeling in the pit of my stomach when I picture them in that moment together. I feel betrayed and the fact that they live together doesn't help ease my worries. Any suggestions or advice on how to move forward?