Seriously talking, I think we should isolate things here...
1. If the Op can afford the fact that this guy will never be in a real relationship with her and could sometime get lost suddenly from her life due to his family, them it's not her duty to protect his marriage.
2. If the guy has the need to cheat, it means that his marriage has an issue - the op is not the reason. His wife might be too stressy, he could be an asshole - anything...