Morbid curiousity
The girl I’ve just recently started dating has a large number of friends in common with me though we did not directly know each other through that circle. That circle knows me and my existing girlfriend and has been under the assumption for years that we two were monogamous. While new girl and I have not made any formal announcements about seeing each other, we haven’t exactly been hiding our affections.
While I don’t see my relationships as social experiments, my fascination with how others around me will discover and react to these events is probably a little unhealthy.
Edit: Everything went better than expected. The friends in common found out shortly after this post and are (at least publically) accepting.