I don't know about cheating, but they are more sexually active than most career groups. This could have to do with relative ages and that RNs are mostly female, so it's a concentration of fertile, young, relatively well-paid women, meaning examining their behavior is easy.
My POV as a healthcare practitioner:
It has a lot to do with spending long hours working in very close quarters with other clinicians, unsupervised. It's also related to bonding over challenging situations, you have no one but each other to share emotions within these situations; again, it creates emotional bonds, which often result in physical ones. Think about it - she's seen you bring people back from the dead, or you've done it together - there's a level of trust there that cannot be achieved in any other way.