If its not in the US Constitution, its a State issue, duh. I don't see gay rights in the US Constitution.
Introductory essays about the Constitution of the United States.
constitution.congress.gov
Supreme court has repeatedly rejected discrimination against gays. Both on theory of "freedom of association" as in the Bill of Rights and also on prohibition of sex discrimination. Some STATES try to claim that their "public accomodations" laws prevail and have failed.
The Supreme Court has recognized expressive association and intimate association under the First Amendment. It has also recognized the right not to associate.
mtsu.edu
First Amendment protects two types of associative freedom
There are two types of freedom of association: the right to expressive association and the right to intimate association.
Additionally, the First Amendment protects a right to associate and a right not to associate together.
Intimate association refers to right to maintain private associations without interference
The right to intimate association refers to the right of individuals to maintain close familial or other private associations free from state interference. Such rights include the right to marriage, the rearing of children, and the right to habitate with relatives.
Some courts place the right to intimate association under the Due Process Clause, but others place it under the ambit of the First Amendment.
Freedom of association often conflicts with anti-discrimination law
A key aspect of freedom to associate is the ability of a group to associate with like-minded persons. Some freedom of association cases have proven difficult to navigate for the courts, because the freedom to associate or not associate often runs headlong into a state public accommodation or anti-discrimination law.
For example, the U.S. Supreme Court addressed the associational rights of the Boy Scouts of America in excluding James Dale, an assistant scoutmaster, because he was gay. The Court ruled 5-4 in Boy Scouts of America v. Dale (2000) that the state “interests embodied in New Jersey's public accommodations law do not justify such a severe intrusion on the Boy Scouts' rights to freedom of expressive association.”