Ahhhhh....Rosie.... I knew you'd talk to me again. The issue is how is marriage defined. Men and women are different, and thus are treated different when the situation warrants it. Marriage has a distinct, cultural, historic, traditional, legal, and/or religious meaning throughout time and place as a union of a man and a wife or man and several wives. Your argument seems to be no restriction, except possible number of participants, among consenting adults. If that is so, why bother licensing marriage at all?
Look, this is a simple equal rights issue.
A man has the right to marry a woman, so a woman should have the same right.
A woman should have the same right a man does, the right to marry a woman.
Can you give me a reason that should not be the case?