Sit down with her and tell her EXACTLY what you said on this thread. Seriously. You can just say, "I know that you're concerned about the relationship I have with X, but I just wanted to say that a) both he and I are very happy just as we are, b) while I appreciate your concern it isn't necessary and we're not going to change no matter what you say, and c) I don't think it's appropriate to be judging me by the standards that you are."
You can set whatever tone in this conversation you like.
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