What is it about stepmothers that gives them an immediate negative connotation?

Im married to a woman who stood in for my kids they love her and she is brilliant weare having our first baby together in september .When i reconnected with a friend the other day i told him i remarried and when i said shes connected with the kids great he said 'ohhh you may think that but she proberly has different thoughts' ive said nothing to her but its been on my mind all week Are some women born with the word "wicked stepmother" stamped on their foreheads, as a form of predestination? i didnt like what he said before we married we were great friends why do people think like this?