I'll add my two cents into the pot, for the record I am a woman.
I wear jeans when riding (riding western) and I have never had a problem getting chaffed, rubbed raw etc except when I was first learning to ride years ago. I never ride for short periods of time, as i've said a few times before I worked for a horse trader for eight years. I spend the vast majority of the day from 9AM to 5PM (7 or 8PM in the summer) in the saddle. I wasn't just plodding around either, the majority of these horses were rank things that needed a LOT of schooling to be solid sellable citizens.
I do not have tough skin, in fact my legs are very sensitive. As a teenager I gained 20lbs in one week after getting a steroid injection to help chronic pain after I broke my femur, I went from roughly 95lbs to 115lbs. As a result I have very bad stretch marks on my inner thighs that are quite sensitive, riding in jeans has never bothered me. The ONLY time I have ever been uncomfortable in the least is days that i'm lazy about shaving my legs and my calf stubble rubs the inside of my boots.
The vast majority of my jeans come from Pac Sun. My jeans have very flat seams that do not stick up at all, even after hours of riding and sweat in all kinds of unspeakable places I can not feel the seams of my jeans as I ride. They are also nice and stretchy, and I wear boot cut jeans that are tight through the thigh and knee, flaring slightly to then go over my boots. I am able to do the splits in my jeans without a problem as well as do functional things such as mounting. I am 5'0 and I can mount 17 hand horses from the ground without a problem in my jeans. I have also never had a problem with my jeans bunching up as I ride.
The ONLY complaint I have about my jeans is that they are not designed for riding and are made from thinner denim. The jeans I ride in need to be replaced more often than my other jeans as they get worn quite thin after a couple of years.
For everyone complaining so much about jeans, what the heck kind of jeans are you wearing so I don't ever accidentally buy some?