Everyone saying, oh you can buy cotton shorts easily, yes, of the baggy variety. And I love boy short pants which give good coverage. But cotton only gets stretch in it and fits tightly to the body if you have either lycra or elastine or some such thing, and the majority of people will go out and buy the cheap cycling shorts from the main supermarkets which are not cotton (I spend a lot of money sourcing pure cotton things, they are not easily found everywhere in the right colours, such as red or green). Cheap cycling shorts on top of pants are NOT suitable wear for a lot of girls and will increase infections- go and ask a doctor how to prevent them, they will say wear cotton knickers and a loose cotton skirt or trousers!
I am hung up on nylon, I hate the fact that in the UK, cheap school uniforms are all made with synthetic fabrics which are not healthy and we have very high eczema rates, lots of kids with bits infections and people wonder why they smell aged 8 after running around in school (earlier pre-puberty accounts for some of it, but synthetic shirts, trousers and sweatshirts on 9 year olds don't help).