I was not adopted, but my biological father died before I was born and I had a stepdad come into my life when I was about a year old... and he hasn't left since (even though he and my mom split up).
I can't even remember a time when I didn't know that he wasn't my real dad. I think they were always honest with me, because it would have been disrespectful to my real father to pretend that my stepdad (a term I only use for clarifying the situation to other people) was my real dad. But for all intents and purposes, in my head, he was never a "stepdad." He was just plain DAD, and still is.
But even though he never adopted me (he offered, at one point, but I felt awkward about it for some reason), I grew up calling him "Dad" and he called me his "Daughter" and that was that. Our relationship is one of father and daughter, period. We have no legal ties or paperwork between us, but 27 years on, we are still very close.
It actually scares me a little bit, because what would happen if he was in the hospital... could I even visit him, or him me? We should probably do something about that.