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".
And conversely, to assert that a human behavior condemned by religion is wrong is merely to assert a religious viewpoint, and that too has nothing to do with the notion of a Constitutional Republic at Liberty to rule itself.
What this issue has everything to do with is individual liberty.
The First Amendment has in it that "Congress shall make no law respecting an establishment of religion ..." And that tells me that no religious code, not even the Ten Commandments, can be forced upon Americans through legislation.
Crimes involving human beings such as murder, rape, robbery, etc. are well covered in American law using a reality based -- common sense -- code of ethics.
When common sense tells people that something is "wrong" -- without regard to some one or another religion code -- then common sense law will cover it.