Question: If Roper wouldn't rent to Jack unless he faked being gay then what made him not suspect the girls of being gay?
Answer: Because they've had various boyfriends and dates over the years. How he never noticed Jack had a stream of women coming to The Apartment is another issue.
Answer: Possibly because in the 70s/80s, no-one would bat an eye about two girls living together, or 2 guys living together.