There really is no double standard. Your question is like asking why are there both men and women, or perhaps why are there two sexes of every thing on earth? That's the way things are meant to be. It would be wise of you to stop believing that men and women are the same, and therefore what a man can do, a woman can do, and vice-versa. I caution you to get a grip on who you are and act accordingly.
NO ARGUING ALLOWED, PLEASE LEAVE NOW!