"A few possibly used condoms"? Used for what? finger puppets? Transporting Red Bulls? I'm not asking you to start genetic testing the contents, but semen has its own, je ne sais quoi... its own look and slimy feel. If you really want to get to the bottom, I would possibly assess the contents of the condom. If it looks like cum, it might be. Of course I'm speculating, I wasn't there. I wasn't invited.
 
You didn't specify the age of the bros in question. My guess is they are under 20, maybe even under 14, and were goofing around with a bunch of rubbers either supplied by a pharmacy bowl o' condoms or handed to them at an outdoor festival.
 
It's unlikely that a Call of Duty hang was the decoy activity for a teenage gay gang bang. Though I do hope any of you reading this name your next band: Teenage Gay Gang Bang. If guys this age are going to have anal sex, it would probably be a more discreet one-on-one scenario. The chances that a bunch of gamers, suddenly said "screw this game, let's all f*ck" is sort of slim.

The condoms, were probably used to whack each other, fling at each other a la rubber bands, and generally creep each other out with -- in typical sexually charger/quasi-homophobic youth fashion.
 
Dollars to bro-nuts, the only thing being worked down in your basement were lame jokes and controllers.