That's a strange line to take. It's a bit like saying a woman can wear the skimpiest bra, and bare most of the mound of her breasts, as long as she doesn't show the nipple and yet men can show nipple and no mound and no one objects.
Of course, the exact rules may vary from one employer to another and they in turn may refer to laws or guidelines within the country of operation. In the UK, during the time the obscene publications act was not only in operation but also taken seriously, AFAIK penises were fine as long as they were not erect.
Where I work, if I ever go into the office, one doesn't have to sail that close to the wind to get an objection - a poster of a woman in a bikini on the office wall would not be acceptable, not because it is is rude but because it is not in keeping with a professional environment. On the other hand, if one was booking a holiday and that same image appeared on a hotel website I doubt anyone would make a fuss.