I have only read the responses on this page.
I told them years ago and took it back recently. The maternal side knows not to mention it or my ex. The paternal side knew about the relationship and the end, but they learned today what happens when they bring up a forbidden topic. DH's family now views us as a mono couple. He is mono, and they hated me flaunting (not hiding) my extramarital relationship. It caused some serious problems. We are still feeling the undercurrents. If DH ever reaches a point of tolerance again and I ever want to get in to another relationship, he has made it clear that he refuses to be out to family, friends, or co-workers. I cannot force him to be outed because of formerly my poly tendencies, so I have no ground to argue with him on this.
I have sincere regrets about coming out to our families. If I knew now what I knew then, I would have stayed in the closet and kept it to myself. Dirty little secret, disrespectful, or not, it was not worth the hell, strain on most relationships, and rifts created. Lesson learned.