Office Romance Gone Awry

So here's the deal, I'm in this open relationship thing with Sarah since we both agreed monogamy isn't our thing. It’s been going well for a while, but lately it feels like it's all I talk about to my friends and coworkers. But anyway, last week things got weird between me and Mark from accounting. Mark's always been friendly, kind of the goofy guy type who keeps his distance. We’ve never had any issues or even flirted before. But one day after work, he starts asking me these really pointed questions about my relationship with Sarah, like if she’s jealous and stuff like that. I mean, it was just so out of nowhere! So the next day at lunch, we kind of got into this playful debate about open relationships. I had a few jokes ready for him but instead, he seemed to be really taking me seriously. Long story short, after work that day, Mark suggests we “talk” and ends up inviting me over to his place for some wine. Yeah... long story short, things got heated and well... let’s just say there was a definite shift between us two. I mean, it's not like Sarah wouldn't understand or anything since she's the same way about dating, but now everyone at work is going to think we're an item and here I am feeling weirdly guilty for something that wasn’t really planned at all. So yeah, just wanted to get this off my chest. Any advice on how to handle things with Mark without pissing Sarah off too much? Or maybe it’s okay and I’m overthinking it.