"Never had the problem before, so I guess my style of riding has changed since I was 12. My friend used to ride in shorts/half chaps all the time, and now she can't without getting rubbed. What you may scoff at as being purely "looks or fashion" may be extremely practical for some people."
I guess my question is what are you & your friend doing differently, and why are you doing it?
On a serious level, is there something about how SOME people ride that results in chaffing and irritation in loose clothing when OTHERS never experience a problem? And if so, is it the clothing that is to blame, or the riding?
The only time I've had problems with stirrup leathers rubbing or pinching was with 2.5" Australian leathers when learning to canter.
Folks can wear whatever they want. I'm all for comfort. But the idea that very tight clothing is a REQUIREMENT seems a big stretch to me. There are just too many others riding English/Australian in jeans or sweatpants without a problem. If jeans CAUSED rubbing or chaffing, then my legs would have no skin by now.
I also don't think that it's a requirement necessarily. Yeah, you can't show up with jeans in the dressage ring and expect to do well. However, there are many other non-horse venues in which ratty jeans aren't appropriate. My old trainer would (and did) definitely give me lessons while wearing jeans, but she considered it a sign of respect to put an effort into wearing breeches and having a somewhat neat appearance. It shows that you care and are trying.
Like I said, wear whatever you want while riding! I don't think that anything is ever a requirement when riding for fun, and I can get by just fine in jeans or sweatpants, but I like to have my half chaps. Heck, you can ride in a women's swimsuit if you want. However, I'm think that you would consider pants of some sort a requirement after that ride!!