Don't be daft, only in recent times has marriage stressed love and companionship more than practicality and 1 male/1 female. I'm not appealing to your sense of what marriage traditionally is, I'm telling you that historically, marriage has been around for people to get married, have kids, and raise them. Find me some figure from the 19th century that thought that marriage was only about love and companionship, and that gay people raising kids would fit into their model of a "traditional" marriage.
When you say that marriage is about love and companionship, I'd agree, but I happen to think that in the interest of keeping the traditional spirit of marriage alive (long term relationship, raising a family) that we should continue to draw the line at 1 man and 1 woman. It's only very recently that any country has tried to change that. Most cultures that accept anything different do so for religious reasons.
You say the fallacy of appeal to tradition applies because we're talking about the legality of marriage. No. The government has nothing to do with the meaning of marriage, it's rather unfortunate that they're involved at all, though I suppose it is necessary. Still though, the government should be able to see that if today it's gay couples, 100 years from now it will be a man and his dog, or a dude and 10 of his crack whores or whatever. From a legal standpoint, you can safely assume that the traditional marriage is the setting that most families will be raised in, because it's also been proven to be the best setting for children to be raised in. The government is only in this because it makes it easy to keep tracking of who is responsible for who, for monetary/insurance/crime/etc. reasons.
But I'm not even making that argument. The idea of marriage is so fundamental to our society, in fact our society in many ways hinges on it, and most other societies are even more dependant on it. When you meet someone, eventually you're going to ask, are they married, do they have children, what do their parents do, etc. When you get vacation time for Christmas/Thanksgiving/summer/Veteran's Day or whatever, most people are going to spend it with their biological families, if it's at all possible. The idea of everyone having a mom and a dad is so ingrained in every culture around the world, even American culture, that I think it would be a mistake to destroy that. Allowing gays to marry would not simply destroy it no, but it would start the degrading of it, over time.
You say how is gay people raising kids against the spirit of traditional marriage? I ask you where in your family tree do you see a John and Bill adopting your great grandmother from an adoption agency, or Sue and Wendy raising your dad after being impregnated from a sperm bank. Does that sound like traditional marriage?