Are Stepparents real parents?

This is sort of a personal question and all but I had a step-father whom I never considered a "step" at all. I never really knew my biological father so my step-dad was all I knew. Now I find myself in a similar situation. When I met my husband 41/2 years ago he had a 3 year old little girl who never knew her mother and she really took to me. I love her with all my heart and I couldn't love her anymore if she was biologically mine but I was wondering, why are step-parents considered lower than the biological? I was 18 years old when I decided that I wanted to be a mother to this little girl. I have devoted my whole life to her and my husband, and that makes me less of a mom than the idiot who decided she wanted nothing to do with her 3 month old baby and would rather strip and do drugs? What are every ones thoughts on what makes a parent a parent?