Do they deserve more respect than some acquaintance you barely know?
I would be more likely to make it an issue with my parents precisely because they are my parents and not some vague acquaintance I barely know.
Let's take another example. Let's say the acquaintance / parents' issue is not with unmarried couple but with bi-national couples (my SO and I happen to be both) or with same-sex couples or with mixed-"race" couples.
With an acquaintance, I would likely let it slide or just go to an hotel but I would also be likely to distance myself from the racist or homophobic host. When all is said and done, what do I care about their bigoted views? It won't affect my life much.
But if it were my parents, I would be more likely to take a stand and try to get them to accept my relationship as equal to that of same-nationality or opposite sex or same-"race" couples. We are not talking about a one-time thing anymore or about people who don't matter. We are talking about things that could be an issue for the rest of my life. I don't want to just distance myself from them. Ideally, I would like to be one big family. If it isn't possible though, my partner is my first family member and my first responsibility.
That's not to say that I would make a big stint. In your hypothetical situation, the hosts were respectful in the way they phrased their request which would go a long way. Depending on how my partner feels, I would likely comply a few times and maybe broach the subject gently and see where this leads.
I am also very interested in whether your view would be the same with my modification to your situation? Would you also think that the guests need to be gracious and should not even think that their hosts are wrong or interiorly resent them if the issue was not the couple being unmarried but being bi-racial, same-sex...? If not, why?
The question is not if you agree with the view or not. The question is not if you find the view ridiculous or not.
But that's precisely the question OP was asking. He did not say "I think children should force their parents to accept them sleeping in the same room as their SO. CMV" nor "I refuse to pay for a hotel room when visiting my parents. CMV".