Everything that they want is available to them NOW, legally, through a lawyer. On one new program tonight, one 'gay' couple mentioned that it cost them $700 to 'legally' change their names. (And it will probably cost them $1,000 to unchange them when they grow tired of each other.)
You raise an excellent question! Why? We can only conjecture as to their motives but we already know the reality. By redefining marriage, they will legitimize what God has denigrated. The homosexual lifestyle is 'unnatural', pure and simple. They can 'pretend' that what they have is a marriage, and do it through the legal system but a 'homosexual marriage' can NEVER be a real marriage. Homosexuals rely on others in order to reproduce - be it through a turkey baster or a legal adoption. God, in His infinite wisdom, gave us marriage. And it takes a man and a woman to form a true marriage.
Actually we don't need to conjecture - they have a stated and admitted objective to change society into one of their liking - with no rules about morality, an anything goes sexual orgy type atmosphere, so whatever outrageous or unnatural think they can think up will be acceptable. If you haven't read any of scripter's links, you should check out his home page and do some serious study. There was a book written in the 80's - "After the Ball" by a couple of homosexual activists, and their stated plans for changing society are eye-opening. The gay activists have achieved most of their goals by now.