I could not imagine wearing any sort of undergarments with sweat pants!
When I am dressed, I wear sweats in cold weather, running shorts in warm. Regardless, I will be commando; I don't even own any undergarments!
Sweats are the perfect thing to go commando in just because they caress your crotch so nicely. Plus, they absorb the sweat from that area much better than do jeans or most other pants.
So, hang loose in sweats!