Tornquad, here's the thing about women... we are nurturers (most of us, anyway). That means that we want to find the best in everyone and protect it as fiercely as we would a pot of gold. We try to see the best in the people we care about, and sometimes we don't even have to try--it just happens. That is half of the equation.
The other half is that seeing the worst in others is simply too painful for us sometimes. I am struggling very, very hard to get over someone who treated me like total garbage. Although he tried to hide the facts from me (and did a pretty good job), I didn't try all that hard to find them. I wanted to see the best in him, and he did his best to masquerade as a normal human being. He did a good job. It is SO incredibly painful to admit to myself that he never cared for me in the least. I am left to deal with what that says about who I am and what I am actually worth. That is a tough thing to examine and avoiding it is much easier.
Your wife already knows her value to you. Isn't that all that really matters? Apparently, it is to her. She doesn't need to waste her time admitting she dated a complete loser a lifetime ago. He is so unimportant now that he is a speck on the windshield. Why not give her the luxury of looking at her future instead of the past that she has overcome? Doesn't she deserve happiness? I can assure you that if she loves you, her dismissal of this guy's actions is not out of affection for him but is instead about focusing on what she has now--YOU.
She forgave him, so you must as well. Look at it this way--he lost BIG here. Be glad that he did because how else would you have claimed the prize? If you love her, you must see how stupid he was not to see what he could have had. It is incredibly lucky for both of you that this man was so blind. It brought her one step closer to you.