I can see why this would make you uncomfortable.
This would be a hard situation to be in. If I was in your shoes I'd stay out of it. You probably don't want to be dragged into all of this. If they are so obvious about their "relationship" for one person to find out chances are others know too. Eventually one of their spouses is going to catch them or hear about what is going on through the grape vine. Either way it is their business let them deal with it. I can understand that this might be hard to do based upon the circumstances involved. No one likes to see anyone taken advantage of or for that matter, to stand by why something you feel is less than proper is going on. As far as I can tell this is not any of your business unless you are one of the parties involved or one of their spouses. I'm sure eventually things will get resolved one way or another, hopefully sooner rather than later. Good Luck to you.