Columbia subject headings:
Homosexuality--Law and legislation.
Homosexuality--Moral and ethical aspects.
Homosexuality--Philosophy.
Same-sex marriage--Law and legislation.
Same-sex marriage--Moral and ethical aspects.
Same-sex marriage--Philosophy.
Harvard subject headings:
Homosexuality--Law and legislation
Homosexuality--Moral and ethical aspects.
Homosexuality--Philosophy
Law--Philosophy
Same-sex marriage--Law and legislation
Same-sex marriage--Moral and ethical aspects.
Same-sex marriage--Philosophy.
Princeton subject headings: