What is a seemingly innocent question that is actually really insensitive or rude to ask?

I was very close to girl/girl couple when we were about 20. One of the girls was my very close friend, and had got kicked out when her parents found out she dumped her high-quality boyfriend for a girl. Anyway we lived together in one room (they had nowhere to go so lived in my private dorm room secretly) for a couple of months, and I could not believe the harassment they endured. This was 20 years ago but still.. the personal questions. We were on vacation at a resort and random people would just be like "so how does he-she fuck you?" like it was nothing. These were otherwise friendly people that just thought it was ... amusing? People seemed shocked when the answer was "mind your fucking business" or "the reason no one is asking about your sex life isn't only that we don't want to picture you naked, but also that it's fucking rude."

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