This is what we Aspergians do....see the logic in everything, and not see the emotional aspects of things.
It's not "bad" to want to diagnose your significant other. You mean no harm in this----but your significant other might feel like your diagnosis is not "objective" enough, or maybe that she just wants to hear it from somebody with a professional knowledge of it, or maybe she, in her heart, doesn't believe the diagnosis, or she merely doesn't want to "hear it," and wants to be in "denial."
It would be hard for me to "back off," truly. I've made that mistake many times.
But the best thing to do, most likely, is to "back off."