It's gross to masturbate in front of others.
"Gary and some friends are playing a game of ""Dare"" when someone dares Greg to jack off in front of everyone."
Gary wants to avoid looking weak in front of everyone.
"Gary tells them that if he did it they would all be embarrassed because of his ""size""."
Gary accepts and jacks off in front of all of his friends.
Gary's friends laugh and move on to a different dare.
Gary's friends are disgusted by it and use the incident as blackmail against Gary.