Just a hunch, but I think it’s homophobia, or maybe just sex-o-phobia. This woman lives directly below the unit in question, so I’m sure she hears everything. And it may just be the noise of people clomping around that’s bothering her. It’s just a thought. I personally would be very bothered if my upstairs neighbor was taking in boarders.
Coincidentally, there is a little studio apartment below my living room where I host my Airbnb guests. The woman living in it–a very nice, reasonable lady–is almost as sensitive to noise as I am. She’s commented before about snoring and heavy footsteps. I’ve apologized and she seems to be able to tolerate the occasional bit of noise. However, it would not work if I took in couples because Blanca would definitely not tolerate it and I wouldn’t blame her.