It’s easy to conceive of socks as being members of a pair, since we have two feet and each sock is its own separate entity. Meanwhile, I can’t say I’ve ever encountered an underpant as detached from its pair. I’m not even sure what underpants are supposed to be pairs of. Are you seriously telling me a single underpant would just be one leg hole? I’m not even going to attempt to visualize such a retarded garment.
As a rule of thumb, if you can’t separate something, it’s not a pair. I don’t really care how many sides of your ass it covers. Like we don’t call a hat a “pair of hats” just because it’s placed above both your ears. Nor should we should call a pair of underpants anything but an underpant. From now on, if you say you’re wearing a pair of underpants, you’re wearing two separate underpants, one over the other, like a little baby afraid of making an accident. Also, if you say you’re wearing a pair of hats, you’re only wearing one hat. I’m pleased to announce that hats will now be called pairs of hats.