Sexually attracted to my straight friends butt

I'm literally in love with my best friends butt. I'm BI and literally obsessed with his little butt. It started a few years ago when he sat on my face joking around once and I immediately became hard with his sent. He's my best friend and I'm upset with myself. I know he has absolutely no interest in me sexually as he's now happily engaged but I still rub it as often as I can ha.