This writer has a Biblical take on morality and homosexuality, and uses the "unrestrained liberty being bad" argument to deny liberty to homosexuals.
Our nation was principally founded on ethics based in reality, and any attempt to impose Biblical "morality" means a return to Dark Age thinking.
Among other things, women would be totally subservient to men, women dress codes would require covering from neck to toes, and all family income would be the property of the male in charge. It would take us back to "The good old days" of a Patriarch age.
By having the "liberty" to make their own laws, the "people" could create the society that they favored, based on whatever values they deem appropriate. The Constitution of 1787 constrains such a society by asserting certain rights, and refers to others "reserved to the states, or to the people".
The "liberty" you refer to - acquiescence to certain sexual behavior - is nowhere enumerated, or even obliquely referred to. Such behaviors would rightly be seen as being constrained by the larger society, expressed through laws. To assert that any behavior is a right is merely to assert an anarachist viewpoint, which has nothing to do with the notion of a Constitutional Republic at Liberty to rule itself.
As far as "ethics based in reality", what reality would you assert that would give rise to such ethics? Natural law, for instance? If so, provide an example of human reproduction by androgeny or parthenogenesis. The social interest in ritualizing heterosexual relationships is to establish a framework for the perpetuation of the society - not merely to provide a legal vehicle for cohabiting adults. As such, the ritualized version - heterosexual marriage - has more basis in "reality" than the notion of trying to legitimate homosexual relations by claiming some sort of naturality for them. Homosexuality is an anomaly observed in some species, and all species have anomalies. Anomalies typically are not a very good basis for asserting an "ethic based in reality".
I agree that the writer has used the wrong reason to arrive at the correct conclusion.
"Gay Marriage" is not about rights. It is about money. If you look at the original push by Gay Rights activists, it was all bout being able to cash in on the benefits that married couples have, but single people don't All those innumerable things that our great nanny state put into law to encourage people to become married. Insurance benefits, military housing benefits, tax benefits, Socialist Insecurity benefits, medical benefits, membership benefits, and all the other innumerable things that THE STATE used to help lure people into marriage.
The one thing that made any sense of all this was the oft stated purpose to insure that "the children" were cared for and raised to maturity.
Now Gay couples want in on the money tree. They already can make all the civil contracts they want for protection of their significant other and to transfer property after death, but they are not satisfied with that. They insist that they loot the public treasury of the resources that were meant to protect children.
What we should have is separation of marriage and State. Let the Churches marry anyone they choose, but have the state only enforce civil contracts between consenting adults with full knowledge or what they are signing beforehand.