I think you are missing my point. We both know that they are trying to keep out the bigger bodybuilder type guys. Anyone can sue whether they are a "protected class" or not. Win or lose when it does happen it will be bad for them either way.
It will get tossed out by a judge before it ever sees a jury. Juries decide disputed matters of fact, but judges decide matters of law. As a matter of law, you can discriminate on any basis you want,
except: race, religion, national origin, age, gender, disability status, and (in some places) sexual orientation. You can discriminate against poor people by charging a $20k initiation fee, you can discriminate against smokers by not allowing smoking, and you can discriminate against the fashion impaired by putting a dress code in place. You can even discriminate in your pricing, on the basis of things like volume or other purchased services. Discrimination based on things other than membership in a protected class happens every day, and is completely legal and not actionable.
Hope this helps.