Need help finding yaoi manga

I read this one yaoi a while ago. This guy has an obsession with butts but was straight before. He notices his childhood friend possibly college student has an a-grade booty. I can't remember the name of it but that friend was actually the reason he got obsessed with butts in the first place due to him seeing his friends booty when his friend fell and showed his booty when they were in a playground. If anyone can remember the name please (づ￣ ³￣)づ