I think Abby's advice missed the point - she approached her response as giving/denying permission, while missing the larger issue of balancing personal freedom and living up to the rules/expectations of those whose roof you live under.

My response, had I been Abby, would have been, "The rule may seem unreasonable, but, you live in their home. They have the right to impose rules in exchange for allowing you to live there. If this rule is unacceptable to you, you always have the option of moving out. At 23, I suggest you look into it."

About the rule itself - I find it unreasonable, but then again I disagree with its basic premise.

If we - for some reason - had 18+ offspring living with us, our "rules" for them would be more along the lines of the rules you'd have with an adult housemate than a minor child. Stuff like rent or comparable agreements and the basic courtesy rules that make sharing a home with other adults without killing them possible. Not curfews and rules about their sex lives.

Honestly, with an adult child living at home, my only sex rules would be to keep it private, quiet, and with legally consenting partners.
"Beware the man of one book." --Latin proverb