How you define yourself has nothing to do with your sexual antics. I was in a very similar situation with my girlfriend and my roommate, and I was worried whether a threesome would change how my girlfriend viewed me and how I viewed myself. Do we have to be swingers now? What if he satisfies her sexually more than I can? If things don't work out will living with him be hell?

We discussed all of these questions together like adults. We made some pretty good points for and against, but it was pretty damn clear at that point there was no turning back. Sitting with them and talking about stuff like "ok what if you like his sex too much?" turned out to be extremely charged, and I'll be honest I can't remember the second half of the discussion because we were all so turned on she pretty much jumped us.