I'm so sorry Steph, I had no idea you were going through that last night.
As a child of divorce with both of my parents re-married, this is how I was raised and grew up being comfortable with: Mom is always mom, no one can ever replace her and I wouldn't dream of calling anyone else my mom. My sisters and I called our step-mom by her first name, as we also did for our step-father. I love my step-father MORE than I have ever loved my dad (long story, that's for another day) and he is more of a dad to me than my dad ever was or will be...but I never call him "dad". I call him by his first name just as I was raised and though I love him dearly and he means a lot to me, he is not officially dad.
I don't know, I guess to me mom and dad are the people who gave you life. That doesn't mean you have to love them (especially if they're say, abusive or something) but they are your birth parents and are therefore mom and dad. That's just my two cents.