Again, even if that kind of thing did happen, even calling it cheating would be a HUGE streatch, unless you expect your widow, to remain sexless for the rest of her life, because you were an idiot, chasing a legend, and got yourself killed, rather than taking care of her.
No, I repeat, you cannot NTR the dead. There is a line in the wedding vows that comes to mind here, "till death do you part". In fact, didn't he even TELL her to go on and be happy? Don't remember the ending of that one too clearly, I would have to go back and look.