I mean I honestly think it should just be a normal situation and the nudity isn't mentioned. Like those times when Marceline or Finn have been bathing, just don't cut away. If anyone gets huffy, ask what the hell the difference is. Everyone takes baths.
But actually the most sensible thing would be to take one of the sex scenes in something like Archer and just don't conveniently cover up the nipples n' bits. I'd love to see anyone justify why sex is okay but nipples aren't. They pretty much have the censors by the nads because they allowed flat-out sweaty coitus.