Most relationships don’t end up lasting. This being said, I know quite a few people who married co-workers and are very happy. Would you deny them their happiness in the name of another Puritanical taboo?
This taboo can be taken to absurd proportions. Two doctors working for different hospitals shouldn’t date if one is a manager, because the relationship might advance (or harm) the non-manager’s career if they ever decide to apply to the other hospital.