If the girl is hot or no one I loved or I only like as a friend then I'll watch granted if the guy isn't fat or weird looking and if they lesbians I'll ask to join unless they related and instead walk away.
I would act natural and proceed to grab my things and act as though they were just sitting on the couch or something normal like that. if it was my roommate that was having sex with a girl, I'd go in my room, head to my laptop and say "Whoops! Forgot I was video chatting with (insert name here)."
Point... Point and laugh. But seriously, probably make one of those stereotypical loud coughs or clear my throat very loudly to make things even more awkward. Then be all like "You done?". Either that or in a continuous motion, accidentally walk in, "ok nope" and walk out, closing the door behind me.... OR perhaps if i was ninja enough, i could sneak out and then back in again my SLR, and then scare the crap out of them when the hear a shutter noise.