Mille (18F) and I (19F) have been besties for pretty much 10 years now, since we were just little girls. She has been like a sister to me and we are obv very close, doing everything together like going to the gym, shopping, etc. We have this one thing where when we meet, we hug and then plant a little kiss on the other one's cheek. It started as something silly she once did to me years ago and we just made it our silly little ritual, like how guys sometimes have their handshakes. Millie started dating Rex (24M) a few months ago, and despite not being very fond of him bec of his age and flat personality, I try to be friendly with him. Yesterday, when I went to her house and we did our kiss thing, he mentioned to me that it's quite weird we do that and how we shouldn't bec it makes him uncomfortable I'm "kissing" his girlfriend. He also linked it to the fact that I'm bisexual. She and I have been besties for so long she's a sister to me and while we are both physically affectionate I def donot view her sexually. I got really irritated and told him that just bec he's an insecure little boy who can't even date girls his age, it doesn't mean I need to change my relationship with my friend who I have known for far longer than he has. AITA?