I remember hearing this story awhile back. It's very strange to me, just going outside of the religious aspect of it for a moment, that people would force someone else to make them a cake rather than taking their business elsewhere. It's like insisting on feeding what could easily bite back. In fact, if the gay couple was really that offended, wouldn't it make more sense to say "There is no way I am offering my business to someone who I think is a bigot. You will not get my money and I don't want your cake."
I side with the businessman on two accounts:
1) I agree with him on the definition of marriage. He's right in holding the definition to a Godly definition, not a wordly one which can continue to change.
2) I agree with him in the classically liberal sense meaning the business is his property and he has the right to do what he wants on his property so long as he isn't violating anyone else's rights.
Had it been a gay businessman denying a Christian couple, I would disagree with the gay businessman's reasoning, but I would defend his right to operate his business as he chooses so long as he doesn't stand in the way of the Christian couple's business.
But I think a real problem is the focus of one group's rights over others. People do have the choice to choose what is right or what is wrong (though both sides will say they've chosen right).