AFER's counter, however, falls short in a way which I feel many proponents of marriage equality fail: by ultimately arguing that "a person should be able to choose his/her spouse."
This argument does not counter NOM's main argument that the government has an interest in sanctioning only heterosexual relationships, that the law justly discriminates against homosexual marriages -- but they're not discriminating against the individual.
I can agree with the premise that marriage law does not primarily discriminate against the individual but the union.
However, is it truly just to sanction* heterosexual marriages but not homosexual marriages?
If a law is a just law and if all of the parameters for that law are met then the law will apply to the entity meeting the parameters regardless of anything else. If a law is a just law and any of its parameters are found to be arbitrary then those parameter will be removed/amended.
For example, if I enter into a business partnership with someone, the laws regulating that partnership relate only to that union – anything outside of that union, in either of our lives, is not regulated or considered by those laws.
In order to define marriage legally, it's important to look at what marriage is, legally. In relation to marriage equality, the question should be: Do marriage laws cover anything unique to heterosexual marriages?
NOM argues that it does, that marriage law protects and regulates the rights and responsibilities of biological parents.
However, marriage law doesn't touch the rights and responsibilities of biological parents. A biological parent's rights are the same no matter his/her marital status.
As AFER stated, "Sex creates children." Marriage does not. Contrary to NOM's argument, the government expects the biological parents to raise their children (or give up the responsibility to someone else) to become responsible citizens, regardless of the parents' relationship status with each other.
If the only difference between a heterosexual union and a homosexual union is the ability to produce biological children, and marriage laws do not regulate the rights and responsibilities of biological parents, then it is unjust for marriage laws to differentiate between heterosexual unions and homosexual unions.
So the counter argument should ultimately be: Homosexual relationships fulfill the legal parameters of marriage in every way; therefore, they should be equally sanctioned and regulated by the government.
*The government sanctions (i.e. supports) and regulates marriage; it does not "promote" marriage as claimed by NOM. Promotion implies proactive encouragement. To my knowledge there aren't any government initiatives to encourage people to get married. Any tax benefits are merely a result of regulating the legally shared assets/liabilities – which can also be a disadvantage (often called the ‘marriage penalty’).