If there is enough support and interest in a SORORITY and your school won't allow it, I say maybe you SHOULD look at legal options. Don't compromise what you want...your campus may have a strong female presence today but demographics may possibly change once you are no longer there, therefore leaving the door open to a situation you didn't think would happen - coed. You shouldn't just back down without a fight if it is something you truly want and believe in.
Remember...well-behaved women seldom make history. I don't know who said the quote but I love it!
