Most Helpful Guy
"Hi. I have enjoyed getting to know you, but I can't allow myself to interfere in any person's marriage."
Does it imply that he's doing something wrong? Yeah, kinda, but that's not the focus. I think the wording makes it about you, not him. I'm not sure there is a way to both give him a reason why you're breaking it off and not at least imply it.
Most Helpful Girl
"It was nice getting to know you, but I'm not interested in interfering in someone's marriage".
Leave it short and to the point, you don't need to make excuses or worry about his feelings. Any negative feelings he has are due to his own actions and if he's been above board he won't have any