I guess my situation is a bit different and more complicated: My Dom is in an open marriage. His wife is very much aware of me and doesn't mind me being with Him, too. In fact, we get along fine. We don't sleep together or anything like that. We both just have our separate moments with Him. It's all very...weird, to say the least, but I'm still adapting to all of this.
Now, if it wasn't an open-marriage and there were specific boundaries in place, then no, I would not even consider being with him. Being cheated on is extremely painful because I had it happen to me before and I wouldn't want to inflict that pain on someone else.