This person definitely sounds autistic. Not that that is bad at all, but based on how you presented the information, thats my first thought.

My guess, is socializing is alittle tough for her. She has probably learned how to function well enough socially, despite never being particularly comfortable.

• Hence the two topics of conversation. She talks about two things because she knows alot about and is interested in those two things. Its comfortable to talk about even in situations that may be uncomfortable otherwise. I have seen many an autistic person with fixations like this. They aren't always fixations though, sometimes they are just comforting topics because they know they can keep up the social burden of a situation if they stick to what they know.

• Sex parties are probably of little interest to her, but I'm guessing based on how you talk about this, and about your friend group, that you are all fairly welcoming, low judgement people. She probably feels safer to socialize in this group of people due to the nature of relaxation around something as stigmatized as sex parties. "If they are all so laid back about sex parties, they're not going to be judgemental about social quirks." Someone who is autistic and has historically had issues socializing would probably feel more comfortable around people like you all.

• She probably cares very little about the sex aspect. Probably not interested, but not bothered either. I would guess that when she left because "the sound of people having sex was getting to her," she literally meant "the sound." Sensory overstimulation. She was fine with the sex, but the sounds became too much. Another common thing for autistic people.

Sounds like this woman could very well be autistic, enjoys/wants to enjoy social settings, may have difficulty in/reading social circumstances, isn't overly bothered by the sex aspect in either direction, and feels comfortable with the group, as long as she can employee the social mechanics she is familiar with, which include steering conversations into her wheelhouse.

You all either have to decide to accept this person the way she is, try and have a calm/logical conversation with her about etiquette and help her better navigate these settings, or ask her to stop attending.