What should I do?

One of my best friends is married (gay) and very much in love with his husband. That said, they don't really have sex. We started sexting a couple years ago and about a year ago we ended up sleeping together. It was amazing and about once a month since we've done it again. Only when his husband is out of town and never at their place. The sex just keeps getting better and I'm not sure where it can go from here. He's a total top and it's amazing. Part of me wants to stop as it can't go anywhere, but we both really enjoy it. I know this can't end well. Thoughts?