If someone is doing harm to another person, that would be action-worthy. But if someone is doing something that harms no one else, it's really not my business.
Borderline issues would be something like excessive noise in a public space, or aesthetic blights, like erecting a massive ugly hotel in a beautiful area. I might object to something like that, even though it doesn't actually harm anyone.
I really don't see how allowing gays to be married can even be considered a borderline issue. On the whole, it seems like a benefit to our shared society. A committed couple is better for society than two singles, isn't that what most believe? I really don't get it.