Widespread discrimination can be a part of traditions though.
Perhaps our forefathers should have called the legal marriage something different, but they didn't. And we have more important things to worry about than people being offended about gays encroaching on their definition of marriage. Marriage legally gives a person the right to be another person's legal relative, and more specifically their closest legal relative and the rights that go with that. This is due to the expected nature of the relationship. What makes that relationship of a certain type that allows for both the couple and society to benefit is not dependent on the sexes of those two people, particularly not in this day and age. We have divorce and many babies being born and raised outside of marriage. We have many people using other means of procreation besides male/female sex. But we also have a lot of laws and benefits and rights that are tied to legal marriage that do affect people's lives.
Plus, on top of all this, we are seeing a relatively new increase in support of homosexuals living free to be themselves, even if there are some people who still feel they are immoral or deviant. (And I say relatively new, because we can look at the line graph of the support for same sex marriage over the last 10 or 15 years, and literally see the changing views represented in that graph. It was relatively constantly at a low level before the 90s or late 80s.)
There are many western civilizations who have already given gays the right to marry a person of the same sex and those cultures and countries are still intact, some doing just as well or better than us.
I'm pretty sure that their were many who believed that allowing interracial couples to be married in the south was encroaching upon their southern cultural beliefs to define that people of different races should not get married. It may not have been a belief held by the majority across the country, but it was a belief held highly within the south (every southern state had a law in place that was struck down by the Loving decision). In fact, it is still a belief held by many in the south. I know, I was raised there and have met many people from there, even young people, who do not approve of interracial relationships. I met this girl in bootcamp who saw a picture of my black HS boyfriend and stopped talking to me. You could read the disapproval on her face. Yet, she got along well with the black girls in our division.