For my two cents worth... KEEP PUSHING THE ENVELOPE, albeit gently, of how much they let you wear the kilt. The more they are around you in the kilt and in public, the more comfortable they'll become and especially they'll see how positive most folk really see it.

Be sure to never be disrespectful to them when discussing the matter, and I imagine two very usefull qualities might be humility and patience. I'd bet good money that in the end if they dont come to positively support you, they will at least respect your interest in wearing the kilt.

As others have mentioned, learn all you can about the culture which has spawned the kilt and why so many men are so proud to wear it (I am assuming you are wearing it for cultural significance at least in some way). A great way to introduce your parents to this is to take them to the Stone Mountain Games or some other such reputable Scottish Fest. There is also the Atlanta Celtic Fest and several good Fests in nearbye South Carolina.

Hope that's usefull... Best of Luck with it. Maybe we'll see you at Burns!