I actually think that's really nice. Even as someone who is quite insecure, I can't stand jealous types who stop seeing 'people' and just see 'the ex' or 'the competition'. If it didn't work out with the first guy they were mature enough to say 'no hard feelings' and get on with the new girlfriend. (assuming he didn't dump her, or they were clearly not working out, rather than her still wanting him because that would be heartbreaking... I've been there, watching the objet of my affection with FIVE other women while we worked/lived together. Awkward, but liking the other girl really helps)
I can also understand the office becoming some people's entire world and they might not date or anything outside it, so they end up whoring themselves around the entire office... can get messy. But yeah if you are always with the same people, and always working, when it comes to fun time wouldn't you look across the desk for the person to let your hair down with, or simply want to get silly with them out of pure curiosity. You've been seeing that same chick with a pinstriped blouse and pencil skirt that makes her ass look amazing for 5 years now... You want to know what it'd feel like to tap that... I guess.
I guess one just shouldn't whore themselves in the first place. Hey, you said sleep with your workmates. Plurally. Outrageous.