AIO for feeling humiliated by my boyfriend's best friend?

I’ll start from the beginning. I used to be friends with a guy, and we were pretty close platonically. We often hung out and had a good time — usually at his initiative. We’d go for walks, talk, and sometimes grab drinks or dinner together. There was never anything romantic between us; I’m sure of that. He always dated a completely different type of woman, and I was never interested in him as a potential boyfriend anyway. He was a good friend who helped me out more than once, but I knew he’d be a terrible partner because I’d seen how he treated his girlfriends. I preferred not to interfere in his relationships because it wasn’t my business, and it wasn’t my place to lecture him. We kissed once, but we were both drunk, and I didn’t think much of it. After that, we didn’t talk for quite a while — more than a year — for unrelated reasons. Recently, I started dating his best friend. When he found out, he said he regretted ever introducing us. He even asked my boyfriend if we’d slept together, and when my boyfriend refused to answer out of respect for my privacy, he just assumed we had. Then he got angry about it. I wasn’t there when this happened — my boyfriend told me later. I don’t understand why he’s even interested in that, or why he got angry. But that’s not all. My boyfriend and I live in different cities because of school — about a 7–8 hour drive, or an hour and a half by plane. He came to visit me this weekend. Our mutual friend got really offended that his best friend chose to spend time with me instead of going out drinking with him. He made it sound like I’d forbidden him to go, as if I were somehow an obstacle to their friendship. Of course, I would’ve been a little upset if my boyfriend had chosen to go drinking instead of spending time with me, but I wouldn’t have stopped him — he’s an adult and can make his own decisions. Now the two of them are fighting. My boyfriend told me not to worry about it, that his friend is just acting irrationally. I get that, but I still don’t understand why he’s behaving this way. I feel humiliated and genuinely upset, because even though he and I are part of the past, we’re not complete strangers after all. I don’t understand why is acting like that. AIO for feeling deeply disrespected by him and kinda humiliated?