Supreme Court Justice Antonin Scalia's argument is, as you say, "abstract and not terribly helpful," but it is anything but logical.
Scalia commits the fallacy of false analogy. In the case of murder, it is not mere disapproval that grounds the law; murder directly harms other individuals and society, so the state must use its power to protect its citizens. Homosexuality and same-sex marriage are private, self-regarding behaviors that do not harm others in any material way. Hence, anti-gay laws rest on mere disapproval (animus), which cannot be a legitimate reason for intruding on individual liberty.
The irony of Scalia browbeating that law student for not appreciating his "logic" is almost too much to bear. It is not the pro-gay rights position that has been reduced to absurdity, it is Scalia himself.