I have some sympathy for the hoteliers not wishing to compromise their beliefs of what is or is not a sin (and I say that without agreeing with them on it), but I'd also agree with the point that's been made that telling this gay couple of the rules when they get there is far too late. Had the website booking page made this overtly and unmissably clear, and had any telephone bookings included this policy explicitly stated as a condition of accepting the booking, then (while maybe still illegal) I could personally see it as a reasonable compromise.
And I agree with Aeon's point that (I would guess) most gay couples wouldn't book at a hotel whose management regarded them as unclean sinners. Why the hell would they want to go there knowing how they'd be seen? If people were going to discriminate against me because of my age, or skin colour, or eye colour, or religion, or sexual orientation, or brand of car I drive, or accent, or .... well, you get the idea .... I'd like as hell want to give them my custom.
But I'll tell you where I think the real fault lies .... the legal system has, repeatedly, made compromises for religious beliefs, and exceptions to the law because of it.
Without wishing to single out any group, I'll give an early and fairly prominent example .... the exception for Sikhs to wear crash helmets on motorbikes because of turban issues. And for that matter, and far more seriously, exceptions to offensive weapons laws to be able to carry a kirpan.
As soon as you start making any exceptions for religious belief, you set yourself up for this type of situation.
And, a belief is a belief is a belief. Maybe Christians that believe homosexuality is a mortal sin and practitioners have a ticket on the Express Train to Hell are right. I'd like someone to demonstrate to me how we prove that assertion, one way or the other, and the same goes for crash helmet exceptions based on beliefs, etc.
My belief is that either the law applies to all, or it doesn't, and that if a Sikh can avoid a crash helmet because of his beliefs, I should be able to chose too. And, if you can make an exception to one law for one unprovable belief, how do you justify not making another exception for another unprovable belief?
For that matter, it'd be a very good start if anyone could actually prove that any religious belief actually has any merit at all, and isn't just a belief. Until then, it is simply a belief and not, in my view, a sound basis for any exception to laws the rest of us are expected, on pain of penalties, to obey.
The hoteliers are not to blame for this mess, and the gay couple certainly aren't. Those that framed the laws, and pandered to unprovable religious preconceptions, however, are.