Yes. This. If my daughter's stepmom posted on a forum and called me biomom, when dd lives with me and visits them for a few hours a week, I would be LIVID. Her father is her *father*, not bio-dad, as much as he irks me and I would love to never have to speak to him or of him again.
In other words, I'd rather someone call me "stepmonster" or "that b***h" (or, for whatever reason, "biomom") or whatever on an anonymous forum, if it means that person is keeping those feelings away from me--and, more importantly, their/my children/stepchildren--in real life.
(That and I don't think most people think it through, nor do I think most people intend it as anything other than as a means of clarification. I do tend to not get worked up about slights that don't really seem to be intentional. YMMV, of course.)