Who knows the actual truth. For all we know the boyfriend is a bigger nerd who had to get a ton of gaul to confront the OP just to ask in a humble way "Is my gf cheating on me? I just want to get the facts, she won't tell me the truth."
Or maybe he was ready to fight, ready to argue but not fight, or maybe the roommate was lying and it was just the pizza guy.
Who the heck knows the truth. The OP has his side of the story, but then again why should we believe someone who is a cheater? :P
EDIT: I take this stance because based on my experience, you never know. I was once in a situation very similar to the OP, except without all the nerd crap or horrible usage of Hearthstone twitter slang (wtf?). The boyfriend confronted me and everything. I stood my ground, ready for a fight- but I am not a violent person. I calmed him down, only to find out the truth. We became friends, and it turned out the girl was manipulating both of us significantly. She wanted me just as eye candy, just to use me for sex/enjoyment. She wanted him to get jealous and get revenge on him. I was pacifist enough to settle him down. He was smart enough to figure it out based on our conversation and his experience with her, and mature enough to calm down and handle it all well.
Turned out she was just a really hot, manipulative girl with a lot of problems of her own. (That's no excuse to treat us that way, but at least she had her reasons for her problems.)
TLDR edit: Girl was hot. Almost fought boyfriend. Befriended him. Turned out she was crazy, but still hot. 10/10 would do again.
Just kidding. If I knew any better, I'd have approached the whole thing differently. Felt humiliated as it was the first time in my life I felt "used". I wanted a serious relationship, she wanted to use me / my body then create drams. Contrary to the sex-fueled "Anything to get laid" redditors we all come to expect, I felt awful being objectified like that. I valued my body/self-esteem and the whole situation sucked. I also didn't even know she was involved, and I aint no cheater bro! lol. I am very glad I didn't go very far with her, although at one point she drew a knife on me and I literally ran sprinting to my car as fast as possible upon seeing it. "FUCK NO I AM NOT DYING OVER THIS SHIT!" 0/10 Would not do again.