The accidental collision of two penises during a threesome, in which the two are aiming for the same hole and collide.
Dude 1: "I was in a threesome last night."
Dude 2: "How was it, bro?"
Dude 1: "It was all good until the penile collision, then all bets were off."
Dude 2: "Eeeeeeeeeeeeew. Gross, man."
Dude 1: "Yeah, I already showered twice."
Dude 2: "You may want to take another, just for safety's sake."