Is it wrong to go on a trip without my boyfriend even if he doesn't want me to?

My cousin is friends with his group of friends. She asked me if I wanted to go to Chicago with them in a couple weeks for the weekend. My boyfriend can’t go because of work. My boyfriend said he would feel uncomfortable if I went without him because those are HIS friends. His friends are loyal and are like brothers to him. I think they’re really cool so it’s not about cheating or affairs. He just think it’s weird if I went without him even if it’s because I go for my cousin.

I asked what if the guys’ had their girlfriends go. He said he’d be fine. this bothered me so I attempted to try to understand his side and ask why it would bother him if I go. He said it’d just be weird that it’s mostly his friends O.o

I was like, “So if my cousin hung out with the group for a night and I decided to go…but you couldn’t go then how would you feel?” he said “I’d be okay with it.” I said “What’s the difference?” he said “Weekend in Chicago is longer”

It frustrated me right there because it didn’t make sense! We just ended the conversation because it was late. I upset him because he told me he was mad at himself now and we’d talk about it later Because he had to go sleep.

I asked this question on another site, loveshack.org and everyone said I’m completely wrong for wanting to go without him because it’s an unwritten code no matter how many times I tried to explain it’s not planned to be a “GUY” trip.