By the same token you could say it's a gun sales law as it forbids people from refusing to sell guns to gay people based on them being gay.
The original law isn't a marriage law, it's a gay marriage law. So you would have to describe your hypothetical as a "gay gun ownership law". But laws other than the one being invoked already allow gay people to own guns.
Furthermore, while you describe the original situation as refusing to do business based on being gay, it's not. It's refusing to do business based on it being about a gay marriage. If a straight person wanted a cake celebrating a gay wedding, they wouldn't be served either, and if a gay person wanted a cake for something else, they would; the refusal is based on the content of the message, not the identity of the customer.
Over at Scott Adams' Blog you can find a very fine example of using the 'Rationality Engine' to solve the social problem of assisted dying.