I can see how ex tensions, emotions, and everything previous posters said *can* (and often will) get in the way of a parental bond between stepchildren and stepparents...but I don't think it's necessarily inevitable. (And I like what anitaj said about if the big love happens, that's great, and if not, that can be good, too...spot on.) In my own situation, one of the nice things I can say about SD's mom is that her non-traditional views of marriage, monogamy, and so forth means she has no problem with me treating SD as my own (whatever that means), because her ideal would be communal child-rearing anyway. She has expressed more generalized problems with me "living her life," but not specifically in regard to her daughter. In that way, I am lucky.
I understand the genetic and evolutionary programming we have to be very attached to our biological mothers, but beyond that, I don't think we're imprinted with "we can only love one mom and one dad, period." We know that's not true--nobody here, at least, would argue that a child with two moms can only be attached to the one who gave birth to her and the other is at best, an "auntie."
But are we programmed to love only two parents, period? I don't know; I'm not a psychologist, anthropologist, or for that matter, a theologian.