It is not an accidental dick-slip. Your buddy has suspected you might be a pillow biter for a while now.  He needs a hole to use and he is not that hung up on it being a female hole.

He just broke up with his girl and is really horny.

He figured now is as good a time as any to determine, once and for all, if you take it up the ass.

You know you shouldn’t, but you can’t help looking up the leg of his shorts. Your “friend” was going commando and his fat cock is fully visible. Each time you glance down it seems bigger. The next time you look up, your buddy is smirking. He just confirmed that he is getting his dick wet tonight.