If discrimination based on sex is illegal why is it that employers can set different uniforms for male and female workers?

My place of employment for instance allows females to have hair length exceeding collar length, they may wear jewellery, and their shirts can be untucked.
Males on the other hand, are not permitted jewellery beyond a watch and (If I remember correctly) a wedding ring, must keep hair length above the collar, and must have shirts tucked in at all times. I'm not sure, but I think females can also wear shorts in the summer.

Every previous employer I have had had similar policies some allowing skirts and opened toed shoes for girls but requiring pants and work shoes for guys in the same position.

It's just something I got to thinking about when some friends where talking about gender equality earlier and it got me curious on why this was legal.