It might boil down to consent? We've learned that elves in general are viewed as little more than cattle, so someone shagging their slave out in the open (with their current "rights" taken into account), as long as the slave is not actively protesting against it, won't be seen as strange or illegal. It might be our world's equivalent of seeing 2 people involved in some "french linguistic training" with their tops off right there in the middle of the street - people might be a little bothered but no one will say a word (unless a cop shows up, in which case, they should run). Over time, as
TheDevian pointed out, people will care less and less to the point where public sex basically becomes commonplace, as long as nothing clearly illegal is taking place.