I think perhaps it's all a matter of pure economics:
18-20 year old women will have guys hitting on them and buying them drinks..which means more $ for the club and more tips for the bartender.
18-20 year old men can buy soda, juice or water. um...and that's it. no incentive or extra $ for the club or bartenders.
besides, when you go out to a club which would you rather spend your time looking at: 18-20 year old women or 18-20 year old men?
hey, in two years it won't matter.