I dont wear underwear under my kilt, never have. My kilts have never been soiled from my body either, any more so than anything else.

I wear my shirts tucked in so the part where the kilt definately touches the body, at the waistband and just below, are already covered by the shirt.
If I am standing then the rest of the kilt doesnt touch my the portion of my body that underwear would cover anyway.
It touches on the legs but not on the butt or groin. Even sitting you only have perhaps two or three more inches of skin touching the kilt than you would have with most underwear.

Its simply a myth that somehow not wearing underwear is going to make your kilt unhygenic. Not wiping your behind might make your kilt unhygenic, but the underwear doesnt have anything to do with that. You are far more likely to pick up germs on the OUTSIDE of your kilt from touching and sitting on everyday surfaces that you are from the inside of your body.
If you are like most people you probably shower or bath at least daily, and know how to use the bathroom properly. If you dont, then THAT is what makes you unhygenic, not the lack of underwear.