Has anyone done this successfully? What I mean is introducing people to a same-sex partner without saying you're gay or bi before hand. Do people ask you to label yourself?
Not in my experience. No one has asked me to label myself yet. I have just said that this is my wife, or I am with a woman. If they do ask you, you can just respond you are not inclined to labeling yourself, but you certainly are not heterosexual! ...or something to that effect.
I asked a girl to prom. I had come out to her with labels beforehand, but not to anyone else at the school. We went to prom together. I imagine that inferences were made. To be fair, I think that some people had already guessed that I wasn't straight; I'm not exactly girly.
Most of the people I'm out to, I never gave them a label. Some of them know I've dated guys; all of them know I'm into girls or seeing a girl right now. They don't ask, in my experience. They assume, and I'm cool with that.