Just because something has been a common practice in a particular society doesn't make it normal or right. Look at things like slavery and prostitution. Both are very old institutions that have been a part of many societies. They didn't kill societies (including the greatest country in the history of the world) and prostitution continues to this day (even legal in some parts of Nevada).
Prostitution continues mostly unimpeded (if slightly regulated) in some of the most "enlightened" societies on this planet. In our country, it's a states' rights thing . . . not forbidden by the constitution.
Slavery went against the capitalist/liberal grain that runs through our conception of ourselves as a nation, hence it could not be allowed to continue. our varieties of capitalism and liberal democracy are not empirically based on universal truth . . . contrary to what some of the founding fathers might have felt. Living by capitalist/liberal precepts is a choice "we" made, and I'm glad we made it, as I'm happy with my life. But I would never assume that b/c it works for me, it has to work for everyone else. I will, however, defend my right (and support those who defend my right) to live by my choice . . .
The same is true of our notions on marriage, the "normality" of heterosexual relationships, etc. They're all choices we make, although we try to act like they were handed to us graven on tablets of stone.
