What makes traditional marriage traditional?

I've been thinking about the phrase "traditional marriage", and wondering how the concept of "traditional" is being applied. I concluded that it must be related to time; the length of a practice and its repetition renders something a tradition.

And so I decided to look at the "one man, one woman" argument, commonly known as "traditional marriage" from the perspective of time.

Turns out polygany (one man, more than one woman) was alive and well in Christianity until relatively recently.

• Paul counseled that church leaders have only one wife, which makes it pretty clear that it wasn't a requirement for regular Christians.
• Augustine and St. Basil of Caesarea both wrote about it in the 4th century. Socrates of Constantinople addressed it in the 5th century.
• Even Reformation hero Martin Luther thought it was permissible under some circumstances, saying:

"I confess that I cannot forbid a person to marry several wives, for it does not contradict the Scripture. If a man wishes to marry more than one wife he should be asked whether he is satisfied in his conscience that he may do so in accordance with the word of God. In such a case the civil authority has nothing to do in the matter." 

• In simultaneous contrast, the Council of Trent took a firm stance on the issue however in 1563, finally declaring polygany and concubinage anathema.

So. Let's ignore the polygany that continues to occur in Mormon circles etc., and call 1563 the beginning of "one man, one woman." That means that what people are calling "traditional marriage" has been the standard for 452 years as of the date of this writing.

I think Abraham is the first biblical example of polygany, and he walked the earth around 2,000BCE. If I do the math, that means that for at least 3,563 years, marriage between one man and more than one woman was occurring among the faithful.

That's 3,563 years of polygany compared to 452 years of "one man, one woman".

Which, then, is more "traditional"?