Since marriage is only between a man and a woman, there's a bit of a non starter there. That said, let's be more accurate here... By legalizing gay shacking up as being equivalent to marriage, it puts the power of the law behind the union and forces others to recognize it, to act with it. If it was just some title, that would be one thing - I could live with disagreeing over it. But to force others to treat them differently because of it, that is compelled action. And that removes my freedom.
Shack up with whomever you want - you do not need my approval or consent. But declare that you are invading marriage and then forcing me to accept it? It isn't going to happen. And to support this freedom, the law will fine and punish those who similarly refuse. Someone walks into a photography studio and says 'I want you to film my gay marriage', you had best drop everything and take this commission, even if they decide they want heavy discounts, lest they turn around and claim that you refused their business because it was a gay marriage. It is not equal rights, it is special rights they claim, superseding any other. You have a right to refuse business, except if someone's gay, then you're committing a crime...
And there's the kernel of why I absolutely object to the corruption of marriage - equality was never the goal. The goal was to use the full force of the government to convert everyone to accept and sanctify their lifestyle. To remove the freedom of choice from everyone else. I refuse.
You beat me to it, and well said.