It's just so difficult to watch your family member and their spouse engage in emotional and sexual affairs, even though consensual. Not to mention be apart of their "lie" to other family members who aren't aware of their arrangement.
Would you feel the same way if your relative were in a same-sex ("gay") relationship and still "in the closet" from certain family members? Would you feel as though you were "part of their lie" if they trusted you enough to come out to you, yet requested that you not "out" them to other family members until they were ready? Would you tell them, "Fine, go be gay, I just don't want to hear about it or know about it, and I don't want you bringing any of your gay friends around the family"?
What is it that disturbs you about (presumably) mature, independent adults doing something that makes them happy and isn't harming anyone?