Here is a man that has no obligation to me. Here is a man that was with my mom for ten years [from when I was 5 to 15] so he no longer is even my step dad by legal definition and wasn't my guardian when my mom passed away when I was 18. However, he's very much a part of my life and my son knows him as 'Papa'.
I've always said 'step' in front of dad so not to confuse others about whom I was referring to when I spoke of one or the other. Although, sadly I must say, I have no connection to my biological father since he passed away when I was five and divorced my mom when I was about three. The man I have called my 'step' dad is the only father I know. And he's an incredibly strong man. We've had our ups and downs but the fact that he stood by me and supports me is a big deal.
I also felt to call this other man my dad was disrespectful not just to my father's memory but to my father's family. I'm quite close to them and I know it hurts them to know that I don't feel connected to Mark. I have actually taken to referring to him as 'bio dad'. Which could be considered insulting since I usually only hear people use that reference when talking about a father figure that was nothing more than a sperm donor. Or in the instance where they were adopted. My parents were together for ten years or so... he wasn't a sperm donor and I wasn't given up. Feels weird.
But I gotta say, it feels good to drop the 'step' and just call this man who raised me 'dad'.