The logic is based upon the aphorism "Begin as you mean to continue", in stating that the first set the tone for all subsequent unions.
But the logic quickly breaks down. Equally valid statements drawn from the story of beginnings (genesis), using the VERY SAME LOGIC, would include:
** The first man married every single woman then alive. This set the tone for us today, insofar as possible. Any attempt to settle for less = compromise and a lesser form of marriage. Or,
** The first man just hung out with critters until God Himself gave him a nap and formed his perfect mate from a spare rib. This set the tone for us today, and any other practice ... Or,
** The first man and woman were as close to genetically identical as possible. This set the tone. Any union with someone NOT a member of one's immediate family is therefore a lesser form. Or,
** The first man and woman wed dressed in the altogether. Any other practice denies the purity of the union and ... blah, blah, blah. Or,
** It was through this first monogamous union that Sin entered the human race. Monogamy is therefore most dangerous. Or,
** You get the picture.
The story isn't told in a book entitled by the author, "Guiding Principles from the Start", but simply "Genesis", the creation. Perhaps an aphorism more appropriate to the minds of both the author and the Creator would be "Ya gotta start SOMEwhere." Or even the Chinese version, "A journey of a thousand miles begins with a single step."
With that aphorism in mind, we are more free to look to the rest of the Bible and see how the Creator's ideas and instructions on marriage developed over the subsequent few thousand years. In doing so, we might well note that when Biblical writers refered to the passage cited above, they inevitably do so in defense of the durability of marriage, not the exclusivity thereof.
However, if our objector wishes to stand by his or her position that the Creation story is PRE-scriptive instead of simply DE-scriptive, they are perfectly free to do so. However, we are equally free to note that to be consistent they must become nudists and live in a forest / orchard / garden without buildings. Anything less is a philosophically mixed bag (i.e. "confusion", i.e. "babel" or "babylon") and of no further particular interest.