I have seen, many, many times, signs at trekking centres and riding stables telling people to specifically not wear jeans. The main reason given is that they're going to be very uncomfortable and rub your legs to shreds given that the inside seam lies right along where your thigh will go against the saddle. Before I took up riding, if I had to get on a horse I would wear light joggers. I can imagine those strong seams wearing out the saddle too.
Jodhpurs are dead comfy, until you try crawling on your hands and knees in them. Then the seam down the front leaves a pretty read line from groin to ankle xD But otherwise they're snug, warm, and feel great to wear.
If you believe everything you read, better not read.