The Argentinian constitution guarantees all its citizens the right to procure their own happiness, which carries with it the right to be treated with dignity by the laws of the country in all areas of life, including marriage.
You don't have to be a genius to figure out that by this logic any kind of consensual marriage is morally justified. It not only justifies two women marrying, it also justifies a woman marrying her stepdaughter and it also clearly justifies a man marrying more than one woman. After all, if a man believes that marrying two or three or four women will make him happy, then he is exercising his constitutional right to "procure his own happiness," is he not?
Unless liberalism is overthrown as the state ideology it is only a matter of time before polygamy becomes legal.
The one virtue of the liberal approach to morality is that it is simple. If it is your desire, and doesn't directly interfere with the desires of others, then it is moral. The alternative is more "intersectional" and therefore more complex for the reason that reality itself has different dimensions, each of which needs to be ordered within a common framework.
For instance, someone might recognise his larger communal tradition as representing an important good and therefore believe that it is important to ask whether a certain action will harm or serve this tradition. He might also recognise the existence of qualities that are inherently good and take these as a measure of the rightness or wrongness of an action. Or he might recognise a natural telos (a purpose or end) to who he is as a man and ask whether his desires are rightly ordered to fulfil this telos or not. Perhaps he has a sense of the sacred in life; of his own higher or lower nature; of what is either noble or base in human conduct; of whether an action produces consequences that harm the individual or the society he lives in, for instance, by leading to crime or instability or poverty or ill health.
Human desires are wayward. We can experience contradictory desires during the course of a day. It is the task of a culture to discipline these desires within a moral framework, one which aims to raise us toward our higher spiritual ends, to fulfil our created nature/telos, and to serve the larger communal tradition we belong to, identify with and love. A society can be measured by the degree to which it gets this moral framework right.