I struggle with the idea of "coming out" as poly because I've never felt the need to announce my relationships to anyone before so I can't begin to imagine how or why I would now. I used to have a picture on my desk of me and a guy from a band that caused everyone who came in to ask if it was my husband. I figured that meant picture on your desk was the universal sign for relationship so I replaced it with a picture of me, my husband and my boyfriend. Oddly now no one asks about the picture...
Where would your co-workers have found out your SO was "cheating"? I guess I would go about spreading the poly information through the same pipeline.