Please stop using "traditional definition of marriage." There isn't one. I'm talking to The Tribune and everyone participating in the gay marriage debate.
If our legal system has roots in the Judeo-Christian tradition, then simply look to the Bible and see the many different definitions of marriage through time: bigamy, polygamy, concubines, marriages that accept infidelity, marriages of very close relatives, marriages between old men and pre-pubescent girls, etc.
Nor has marriage only ever been about one man and one woman falling in love and then marrying to procreate. Sometimes it is mostly about that, but it often has also been about property, titles, inheritance, security, alliances.