I don't know about "plain old sweatpants" but the ones I grab all the time to workout in (that are the most comfortable) are men's PJ bottoms that I bought at Target years ago! They were about $9, with pockets, drawstring, very lightweight material, and no fly! Now, if I belonged to a gym or took classes at a dance or yoga studio, I would not wear these out in public. But, just to workout at home in, they are my favorite! Or, to take out the garbage in, they're just fine. Ha, if that's not a ringing endorsement, I don't know what is?!
I hate heavy material, or workout clothes that are too tight. And, these were just right. Came in traditional solid colors of grey, navy, and black.
I haven't seen them in the stores anymore, otherwise I'd have bought a dozen more. And, no one would know these were men's, since there is no fly! I bet I've had them at least a dozen years, or maybe more?