When a man, gay or otherwise, has anal sex with at least 3 or 4 other men throughout the same day and night. Girls can be count as part of the mud run, but the one making the run is always a guy.
Guy 1: You guys wanna come over this weekend for a BBQ, some drinks and then maybe a mud run.
Guy 2: Ah, yeah, that could be fun. I've never done a mud run before.
Guy 1: You don't know what you're missing. You can even bring a couple of girls along, just not too many.