If marriage can be redefined so that it no longer means a man and a woman but two men or two women why stop there? Why not allow three men or a woman and two men to constitute a marriage?
I had really great parents who always gave me lots of opportunity for choice but I didn't always realize how rare that was for a girl for them to say 'You can be a mom or have a career or do both or do something we haven't thought of yet.'
