Boyfriends friends dont like me very much?

half of his friends like me, and the other group not so much. I treat them as my friends too but out of nowhere i get a eerie feeling they dont like me very much. And they always want they boys night and they get angry when my boyfriend brings me along. Other half of his friends never have this problem and we all get along fine, but its just his friends before from high school. And one of them told my boyfriend too and hes having a bday party on a day me and my bf have already planned 2 months in advance. I decided that since my bf really wanted to go, its fine we dont do our thing. but then i find out im not invited. His friend told me its a guys night out, but i know girls who are going. I really dont know what im doing wrong. Is it bad that my bf defends his friends dislike of me? And I'm starting to feel like he blames things on me so his friends dont get mad at him. but yet he expects me to be there for him after he hurts me. Should I be frustrated? Should i be disappointed that he chooses friends like that?