I can't imagine that anyone with any common sense or decent grasp of the English language wouldn't know that it would be offensive. And it seems that many who use it have an agenda.
I don't use other sites. And I think that it's definitely closer to fact than opinion that the woman who gave birth to a child doesn't need to be described as the 'birth mother' of that child in an instance such as this.
But yes, I understand that some people are wilfully stupid, some are deliberately ignorant, and some make genuine mistakes because they don't think before they speak/type.
I don't think a 'note' at the top of the page telling people things they should or should not say is a good idea. I prefer that the aforementioned people identity themselves to me as such by the things that they say, so that I may avoid them in future. I don't enjoy being told what to do/say because I am not a child. Neither should anyone else.
Sometimes existing users of the boards here can be very quick to jump on new users for making a mistake. If everyone was a bit more 'gentle' in their tellings-off, perhaps fewer people would be upset. Certain boards on MN are especially 'touchy' (for various, understandable reasons), and this is one of them. And certain frequenters of said boards seem to enjoy designating themselves the position of Thread Police.
That is why I tend to avoid such boards!