An event where guys significantly outnumber girls. This isn't great if you're a guy, and is especially not so great if you're an arsehole, as generally this means that your chance of pulling is, well, diminished.
English is a tricky language. If there were more than one cactus in the room, you would call the room filled with cacti. But if there is more than one penis in the room, you would call it a sausagefest.