I stayed at the Ipanema Plaza when I was accompanied by a blond, muscled Argentine who was 20 years younger than me. We were treated in a friendly fashion there.
Late one afternoon, on coming back from the beach together, we did see a man stopped by security before the elevators. He was with a rather luridly and scantily dressed female. He was asked to register his 'guest' at reception before proceeding.
Having spent a lot of time travelling in Latin America, I appreciate the security of having strangers registered, and their identity checked, by the hotel before they go to a room. I have heard of various violent incidents where guests managed to evade security and were then robbed by their newly-met companions.