I think you should tell your wife about feeling excluded, and see if she can include you more. You could have dates with the three of you, or bonding time with her boyfriend, or have her keep you informed of things, whatever makes you feel less left out.
About fairness, well it's kind of a hard question. I don't think fairness is the same as parity. I mean that I don't think you should always have partners at the same time.
Let me explain: if you push it to its logical conclusion, if one of you breaks up with their partner, then the other one has to as well, even if the relationship is going great, even if there is love, etc. That would, in my opinion, not make it fair at all, even though it's about parity. It wouldn't be fair to the two people whose relationship is working, and it would be extremely unfair to the other partner who had nothing to do with any breakup.
This is similar. If she really likes this guy and he likes her, is she obligated to wait until you find someone too? That doesn't seem fair to him and her. It's too bad that you're not finding partners, and it's notoriously harder for males to find females than the other way around, so I sympathise, but I don't think putting her relationships on hold indefinitely is the way to go. I think it has the potential to damage your relationship with your wife, as she would grow to resent you more and more for not allowing her to be with someone she likes "just because YOU can't find someone".
You can however ask her to be understanding of the fact that you don't have another partner, and feel lonely when she's gone, for instance. I think you should definitely find things to do when you miss her (hang out with friends, indulge in a hobby she doesn't share, etc). You can probably find advantages to that time away from her to do things you want to do but don't when she's around for whatever reason.
Make sure she knows how you feel. It's very important, since otherwise you might be more and more upset with her as time goes by and she'll have no idea why.