Have you taken another guy's underwear in the locker room or traded yours for theirs?
In my gym there is this really fit hot guy and he is always in his jockstrap or naked in the locker room. I always get a boner from seeing him and once while he was taking a shower i switched my jockstrap for his. Then i went into one of the stalls and jerked off while smelling it.
Another day i saw him go into a stall and i followed him for curiosity and saw him jerk off through a little crack and saw him dump his load on another jock strap he had. I got a HUGE boner and after he got out and went to the showers i took it and went into a stall and jerked off while liking his still fresh cum of the jock strap. I took the jock strap home with me and i dont know if he noticed but he didnt seemed to look for it. He just walked around naked for the rest of the week