So you've come up with an example that is clearly on one side of the line. Ok, but not sure what that proves. I don't think anyone has argued that there aren't any clear cut cases.
What you are missing is that whatever rule you make, people are going to try to push the limits, because it gets them viewers. If twitch says that someone bending over in a bikini has no non-sexual purpose and therefore is against the rules, the creators could change the stream very slightly.... just enough so they can claim to be about exercise, yoga, modeling fashion, or dance. They'd still have the same audience, with the exact same reason for watching, but now it sneaks by the rule.
Pole dancing is an art and takes athletic skill, sure, but most people (at least in the US) consider it is purely viewed for sexual gratification. But creators could claim its "main" purpose is to enjoy the dancing for its artistic and athletic merit.
Sports Illustrated has a swim suit issue, and it's not about swimming so how is that sports? It's just an excuse.
The majority of art channels on twitch are people drawing sexy (often anime-style) girls and women. You can learn to draw from them, but the sexier they make them, the more viewers they get.
So... I really don't get what you are trying to say. You seem to not understand that there is gray area.