What are you talking about? Care to explain what you mean by "most sports"? And what sports could be mixed? If we look at some sports where athletism isn't the main factor, like shooting, darts or snooker...even in these sports women cannot compete against men at all, so what the hell are you talking about? What obvious thing I'm missing, please tell me.
edit: Equestrian is one of the very few sports where women can compete against men and that's only because the horse plays the most dominant part in the performance and biomechanically the horse works independently.
edit2: There are already rules in many sports how athletes are classified either a male or a female, so this has been already thought out.
Because shooting, darts and snooker are "traditionally male sports". But from footbal to sprinting, the real physical difference isn't that big.