"Love your neighbor" does not extend to "normalize your neighbor's erotic proclivities at the cost of broader society" or "you must erase the distinction between things."

Christians have traditionally believed that marriage is permanent bond between a man and woman for the purpose of forming a household and raising children*, where the duties of the man and woman are asymmetrical. For a man to “marry” another man is a contradiction in terms, the same as when your boss tells you, “I want you to think of me as your friend, not your boss.” The male-female relationship has elements that are inherently asymmetrical, and inherently different than male-male relationships, and different things deserve different words. Furthermore, the male-female union is a core building block of civilization and therefore deserves special recognition by the state and by the church. It deserves to be considered normative. One of the things that has especially turned me against gay marriage is seeing how so many institutions (eg public schools) no longer feel empowered to teach the male-female marriage as being the default or the normative institution. Legalizing gay marriage was not just “allow different people to do their own thing” it was, “change the basic way every child is taught about the basic institutions and building blocks of life.”

Now part of the problem for modern Christians is that many already have given up on the idea of marriage being permanent or that the husband and wife have different roles and obligations. Once those distinctions have been erased, resistance to gay marriage then looks very unprincipled. But for traditional Christians the argument is very straightforward and consistent.