Well you're a guy so for one, don't wear 3/4 lengths pants. Those are for chicks. I don't know what you were wearing, if they were shorts then that's fine for dudes but that brings me to point 2 - most "trendy" nightclubs aren't going to let you in without full length proper pants on. Especially if you're a walk-in. And of course they're going to let women wear what they want into the bar. They're trying to attract people into wanting to hang out there. Most people don't go to a place like that to pay for overpriced drinks, they're there to hopefully pick someone up or at least stare and wish.
Brotha that's just how it is. I'm sorry if you disagree with the current societal norms but if you want to fight the man then the Armani bar doorman probably isn't going to be your first convert. Write a column or a book or a song or something.