I wear plain cotton underwear too. I wear them inside out. This seem ridiculous, but it prevents the seams from rubbing my skin. I also wear them a bit larger than necessary so there is no binding. I wear my socks inside out too. If I ever get in an accident, the medical personnel will think I'm nuts.
I haven't worn regular jeans in years (partly fibro, partly too many abdominal surgeries). I can wear the stretch ones with elastic waists, though. With a long shirt, it's harder to tell that they aren't "real" jeans.