Back when I started, there was virtually no men’s clothing, so they wore unitards and these kind of bloomer style shorts. That was before bike tights became a thing.
I used to webmaster for a site of a very famous yoga teacher. About 10% of his traffic came from a link in an article for, uh, self mouth pleasuring - and the advice was to limber up and a link to his website for some reason. But that’s a lot of traffic. Made me think there was a market there 
There’s a very funny SNL skit about this but I can’t be looking for it at work.
The whole yoga pants as fetish thing is super bizarre to me. But if you google up yoga pants you get a lot of sexy photos. I guess it’s kind of like “reality” p*rn in that it seems like real people and not actors.
Edit to add I do know a few people who do nude yoga as a real thing - but have seen a few videos that make me think that’s not really what most of these searches are about.
