Genuinely, Jesus said getting remarried after divorce is adultery, so should business owners have the right to interrogate potential adulterers and refuse them service if they are deemed to be adulterers? It's exactly the same principle. Should a Christian shop owner have the right to interrogate potential atheists, since marriage is a Christian institution, and to deny those atheists service if they are deemed to be atheists? Should a Christian shop owner be able to interrogate a potential Muslim or Hindu, since marriage is a Christian institution, and thus be able to refuse them service if they are deemed Muslim or Hindu?
All those - atheists, Muslims, Hindus, gay people - are 'sinners' in that they openly do not believe or follow in the same religious tenets as Christians do, and in fact openly oppose the Christian tenets on various levels. Is that not what this is about? Gay people openly go against God's word? Well so do Muslims, Hindus, atheists and adulterers. But I guarantee you, if Christians fully applied this principle of interrogation and refusal beyond only gay couples, they would be out of business fairly quickly.
And to bring up another problem with this viewpoint - that it's okay to discriminate against gays in business - what is being in business? Being in business in a country whose laws cater to the equality of people of all faiths and equally to those who do not have any faith at all, means a business owner is assumed to recognize such business laws. Opening a business is a signatory act, undertaken by the business owner to abide by the business laws of that particular country. It is therefore a legally binding action; to open a business, in that any owner recognizes the authority of the legal precedents set forth in relation to business practice, to monetary laws, to consumer rights and so on and so forth.
A business is, by definition, a commercial entrepreneurship. This is the practice of setting up a construct to gain money in exchange for services or goods; an organization or economic system where goods and services are exchanged for one another or for money.
So, to surmise, a business is opened up as a means of exchanging goods or services for one another or for currency. I'd personally call that pursuit of money. And a business owner, upon opening a business, undertakes by proxy a voluntary agreement to abide by national business laws and practices.
There are two problems with this. Firstly, if a Christian (or any other religious person, for that matter) creates a mechanism for the pursuance of currency whose creation is considered an automatic voluntary agreement to be bound under national business laws that may or may not contradict their religious laws then that person willingly takes a risk of being left in a situation where they are legally obliged to go against their religious laws. That is voluntary, since no person is forced to open a business.
This is in effect 'serving two masters', which is expressly warned against in the bible.
And secondly, if one serves money itself in contradiction to their teachings against it, then denies service to those willing to give them the money they pursue, due to their religious considerations then this is an act of hypocrisy for the owners go against their teachings, long before the gay couple come though the door, by opening a mechanism for the pursuance of money and thus voluntarily agreeing by that act to be bound by the legal constraints that may or may not go against their religious laws.
