As reported here, an enterprising blogger sought to get a bead on what constitutes a “traditional” marriage.  He unearthed a judicial opinion from Kansas from 1886 (and can you get any more “traditional” than Kansas in 1886?), in which the judge wrote:

In my opinion, the union between E. C. Walker and Lillian Harman was no marriage, and they deserve all the punishment which has been inflicted upon them. … In the present case, the parties repudiated nearly everything essential to a valid marriage, and openly avowed this repudiation at the commencement of their union.

What did the couple repudiate, thus making it “no marriage”?

* the bride declined to take a marriage vow of obedience to her husband
* the bride declared her individuality by keeping her maiden name
* the groom acknowledged at the wedding ceremony that his future wife could “repulse…all advances of mine”; in other words, he acknowledged that he could not rape his wife-to-be
* the groom acknowledged that he would share responsibility for child-rearing
* the groom disavowed that his wife-to-be was to become his property

Yup.  So, as pointed out—in 1886, the idea that a woman owned herself, even when married, and had the right to not be raped was a radical redefinition of marriage, so much so that a court refused to acknowlege it as a marriage at all.  Moreover, there is hardly a marriage today that comports to this old-timey view of marriage.

The point, if it is not obvious, is that the societal concept of a “marriage” evolves over time, so it is simply incorrect to assume that there even is a “traditional” notion of marriage.