My Secret Kink as an Engaged Graduate Student

I'm a grad student at a university in my mid-twenties and I've been engaged to Sarah for the past six months. We're both committed to each other, but there's this kink of mine that I know would shock her if she ever found out. To be fair, we have an open relationship. There are certain boundaries set, we never introduce anyone new in our shared social circles and no overlapping with friends or family, but it allows for the exploration of different sexual interests without feeling guilty about it. My kink involves a bit more than just casual role-playing; I really enjoy a strict boss and employee dynamic, but in an office setting. It's all very consensual and controlled. The thing is, this past week, there was an administrative assistant named Mark who started working at the university library. The moment I saw him, I felt drawn to his professionalism, his neat appearance, and that authoritative air about him. There were a few times when we interacted, usually for routine tasks or questions, and every time, I got butterflies. Yesterday afternoon, after an unusually challenging week of research, I finally asked Mark if he was interested in exploring the dynamics I mentioned earlier. He said yes, and we arranged to meet at a hotel room this Saturday. The thought of being 'his' employee, submitting to his authority in that way, is both terrifying and incredibly arousing. So here's my confession: I've agreed to explore something with someone who isn't my fiancée, even though it contradicts the idea of monogamy some might hold dear. But this kink feels too powerful not to act on. Am I being selfish? Is there a better way to balance my desires and keep my relationship intact? What would you all do in my situation?