Well, as a female I'm all for employers banning women showing bare midrifts. I think they're pretty unsightly, especially when an ugly thong creeping over the hipline is thrown in for good measure !
On a practical front, I don't think you're likely to do your career much good if you start trying to be a trouble-maker and accuse your employers of being discriminatory. Surely they are only trying to get all employees to turn up to work in a decent standard of attire? If you're feeling hot and a suit and ties are deemed manatory for male employees, you can remove your jacket, wear a short sleeved shirt, and loosen your top collar button, and also your tie.