1. When two or more guys kiss and make out and get off together. A quick kiss or grope doesn't count. Extended tongue action and fondling are required for an act to be guy on guy. Most of the time this ends when they wank each other until they come (because guys are good at wanking). Guys who are more familiar with each other, or kinky, will 69 and suck cock and rim and have butt sex.
2. Porn for girls. The type of porn in which all participants are hot and generally getting off on the proceedings. Participating in a guy on guy moment is the number one female fantasy.
Julie: Did you get that Mad TV guy on guy youtube clip that I sent you?
Jodie: Omg that was so hot, do you think that all guys really do that?