My children have tons of friends that call me mom. They have friend's moms that they call "Mom" as well. I think it is very similar in comparison. I am not any less their mom than I was before they called the other person by the name. And I actually think it is good for the kids to bond with someone that cares for them and has their best interest at heart.
So why is it such a terrible thing for a child to call another woman "Mom"?