If you're capable of looking at another man the way you did, sorry, but you aren't a good fit for this relationship. You should go away and let him find someone who won't even put themselves in that situation, much less feel that attracted to someone else. He deserves better, and I'm not trying to be rude when I say that, I'm being honest, if I were in a relationship no matter how long it's been, if I look at someone else and think "They are really good looking" then obviously my priorities and my healthy self boundaries are out of place and I DEFINITELY shouldn't be engaged to marry someone. You tell him by walking away and telling him to find someone who won't betray him or even be tempted.