Random, thanks for sharing. Your latest message, you've every right to pursue other relationships. Of course, you need to communicate this desire and evaluate your current guy's reaction; but I cannot see how, in a healthy relationship (overall), he could refuse. After all, he has his wife! He may have reasons, but I cannot see that in the end, long-term, they would be valid reasons to refuse your request.
My own scenario is, or was to start, similar to that of the wife in this story. As a gay man in a 4-year relationship, recently engaged, I didn't expect it, to say the least. And, similarly to the wife here, I felt that I needed to be better friends with the "new guy" (or gal, in your case) so that I could understand, appreciate and love the relationship. I felt that, unless I knew the new guy fairly well, it would be next to impossible to feel compersion (I find that an ugly word; these days, I prefer empathy or compassion, in the PASSION sense).
And you know what. I was at least partly right. Because you know what? I think you can probably look at the wife and see what's good in her. I think you can (should be able to) have a great chat with your guy about what he sees in her and why he (presumably) loves her so much. This DOES NOT mean that it will be stuff you have in common - heck knows. But you might learn a thing or two, and no doubt she can learn some great things from you, two.
I think we discount the opportunity to make new friends too readily. Yes, we may have full lives. In your case, it may help the relationship grow, so it may be worth the effort.
On a final note. If you really can't see good qualities in her, and/or he can't explain clearly to you what those good qualities are ("I just love her, don't question it!" Hmm...), you may need to question your guy's judgment. If he's so in love with a difficult to get along with person - a girl you wouldn't even be FRIENDS with - what does that tell you about him? (Not in a judgmental way - merely in a "is this a guy who is right for me as a major life partner" way.)
Good luck exploring your options, and sending you good vibes,