I don’t know what you mean by you crossing a line.
Think your boyfriend crossed a line by even considering going without you. It’s not like you two just met and you are a casual girlfriend.

You keep focusing on his friends. Who knows what’s their agenda. Could be that your partner shares with them too much like telling them bad things about you and that’s why they don’t want you there. I’d not blame them as they owe you nothing. I’d focus on him. He is in the wrong by not addressing it

Do you depend on him? Is he a good partner overall?