My best friend's bi/lesbian friends.

I wasn't sure what category this would fall under, but I know I wanted to get other women's advice on this. My best friend Josie has 3 other friends who go by the name of Brittney, Amanda, and Lexi. They are all either bisexual or lesbians, which is fine, I don't mind. But have you ever gotten the feeling in a group of setting that you are the black sheep of the lot? You just KNOW you don't fit in? That's how I feel with them. So I try to steer clear, so nobody is uncomfortable or feels obligated to include me in activities. Whenever josie has a get-together or a party,  she invites me and doesn't tell me she invites her other friends. I feel bad she wants us all to get along and be buddies, but I know I don't belong. I'm the only straight one there, and there is an invisible barrier because I'm almost positive she's had sex with all of them, herself being a lesbian. What do I do 😫 I feel so bad.