Well women's beach volleyball, they are practically wearing lingerie. Sure it's cooler and skin tight helps keep the sand out so that one I think is practical.
I've noticed women have started wearing leggings into office settings. So now there is no real difference between the workplace and yoga class, at least from the waste down.
If it's about practicality then why are the guys not in skin-tight stuff too? I'm sure the optics have a bearing on things.