Step brother problems

I have a step brother who is on the spectrum and I love him. He's great. We are actually pretty close, but my friends made our relationship weird. Me and him were play wrestling when my friends were over and he got a boner. He either didn't notice or it didn't bother him. My friends saw and since then they've made jokes about us sleeping together and every time we are together they say "What are you doing step bro?" He doesn't understand what they're talking about but I do. My friends have sexualized our relationship so much it makes me sick to be around him and I hate it because I know me avoiding him hurt his feelings.

Edit: My step brother is 15, I'm turning 18 in October.