The extension of certain essential rights--joint tax filing, hospital visitation rights, bereavement leave from work, to name a few--to mutually loving, consenting couples, regardless of sexual orientation.
Its legal ban is part of President George W. Bush's bid for the Bible Belt's vote in the 2004 election.
We shouldn't allow gay marriage because gay couples cannot reproduce! In fact, elderly couples can't reproduce either, so let's nullify their marriages! And while we're at it, let's ban sterile people from marrying too!
If we allow gay marriage, fairies will come from La-La Land and magically turn everyone in the United States gay, stopping us from reproducing! So ban gay marriage!