Well, your traditions went out the door with divorce laws. There is hardly a family that is 'a man, a woman and children' shucks, there are enough couples who are - by choice - without children. Not to mention enough 'families' who are without either man or woman...
Neither divorce (which I was once) nor being childless (which I am) changes the definition of marriage.
You are keeping an argument alive that is still stuck in the last century: Up until the 1960s or even 70s a woman was to get married and have children. That was her sole purpose in life. But we have since moved away from that. I think those sentiments, that a woman can actually have a job outside the home was also met with 'you open the door to...' arguments.
I disagree. Recognizing the right of women to vote, own property, be educated, and have a career has not opened to the door to anything else, because there isn't anything beyond that; every person enjoys those freedoms no matter what kind of relationship they find themselves in, as they should.
And my argument is *not* about the joys or pains of non-traditional marriages; read my subject line again. My argument is that same-sex marriage proponents claimed that recognizing same-sex marriages would not lead to demands for polygamous marriage. They have. Ta-da.