First, it was a question, not a statement. It was a request for clarification about what they really plan on telling this child. About what they plan to say if she presses for more information about what 'girlfriend' means when he clearly already has a wife. I think that's important.
Second, in this day and age, 'girlfriend' usually does mean someone you're having sex with. Not always, obviously. My experience--on forums, etc--has been that it's so common that many people tend to get very critical of people who choose not to have sex with their boyfriend/girlfriend, and act as if that person is abnormal or perhaps has some issues. Based on reading on this board, I don't think it's weird at all that someone would think most people on this forum in particular are having sex with their boyfriends/girlfriends. (Might be an interesting poll.) In fact, a comment addressed to me just last week made the assumption that I am 'fucking' my boyfriend and no one at all stopped to point out that it was, in fact, an assumption, that I have never once said I'm sleeping with him.
But the fact remains, it was a question, not a statement.
My comment was not on the value of open-mindedness or tolerance or teaching it, but on being so eager to exhibit one's open-mindedness that one races into a conversation sooner than might be appropriate, or discusses things that aren't necessary.
I don't find it demeaning at all to introduce someone as a friend. And if my boyfriend had children, I would find their well-being more important than what word was used to describe me. He himself puts concern for my children (and how my dating a married man would affect them) above his feelings about whether I introduce him as a friend or boyfriend. I believe children's well-being should always come above adult's concern about a label. But everyone has their opinion on that, and that's the value of asking a question here and getting different opinions and views on which to reflect.