There's no need to re-define marriage for a small subset. Marriage is a religious term, not a legal one. There's a lot of confusion and overlap, but a couple can be married before God and not legally married, and vice versa. Religions should be free to determine their own definition of marriage in accordance with their beliefs, and legalizing gay "marriage" would likely lead to discrimination lawsuits against churches who refuse to facilitate or recognize the wedding/marriage.
That being said, gay couples should absolutely be allowed all the legal privileges associated with marriage. Ideally they would be able to get civil unions (for those who think it sounds too cold and legal, make up your own word for it - just don't go re-defining the terms we already have).