I still don't see any issue with this if no one was harassed, pressured and it was a case of two consenting adults having some fun. They didn't report directly into each other during the day (after hours was a different story ) so what's the big deal?
Actually I have an issue with the fact that some were in relationships, but that's an issue for their respective families, not the workplace.
I'm pretty sure there was an article last year about how many relationships start at work and it was around 30%! We have quite a few in our workplace and it's not an issue at all.