Wondering if the issue is not wanting to wear anything 'under' the kilt...or not wanting to wear anything between their legs to minimize rubbing or chafing?
For me it's the latter. It's not so much about rubbing or chafing as it is about being restrictive. As long as 'the parts' can get air and are free to do as they like without being cramped up, I'm good. Nothing's worse than sitting down in a pair of trousers (or even just a pair of boxer shorts) and having the seam try to split you up the middle. A kilt liner would definitely fit the bill for me.