And if they wear them correctly, they wear nothing underneath.
I'm afraid I couldn't wear a kilt for fear of an inadvertent woody. Knowing my junk was banging around down their without any restraint would make me think about my cock, and then just thinking about my dick would make me get hard. The sensation of the wool fabric rubbing on my glans would also get be hot and bothered.
My mind would start to wander to wondering what my boyfriend would look like in a kilt, what it would be like to give or receive a blowjob wearing a kilt, and then pretty soon I'd be poking out like a flagpole. That's why I need to wear jeans, to keep my friend under control in public. Just thinking about wearing a kilt and getting a boner is giving me a boner.