Homosexual agenda advocate: What do you conservatives care? How does it harm you? Why are you trying to control other people?
Conservatives: Here are 352 examples of the fact that when this stuff is in place, your side gets to control other people--namely, people who think like me--and punish us for not agreeing with you. That's why, just for starters.
HAA--Well, those are all _reasonable_. If people choose to deal with the public, they should _have_ to go along with this. And when we've passed homosexual "marriage," then it really _is_ legal marriage, and it's just acknowledging _reality_ to call it "marriage," so people should be forced to do so.
Conservative: I rest my case.
To be clear, according to McGrew, same-sex marriage is harmful to the conservative because if same-sex marriage is legal then the conservative will be forced against her will to acknowledge that it is technically true that same-sex marriage exists. There's harm, and then there's harm.
* I realize it's not that simple. You can't argue that e.g. slavery should be legal as long as you have the right not to sell yourself into slavery if you don't want to, and if somebody else wants to sell herself into slavery then you should mind your own business. Certain freedoms, such as the freedom from slavery, ought to be protected no matter what. But that idea is instructive in itself.