Scripture tells of two types of crimes: crimes against other people and crimes against G-d. (There might also be crimes against animals or against nature, but they are really also crimes against G-d.)
G-d (presumably) forgives all crimes, especially Christians believe that.
Crimes against other people are also crimes against G-d, but they require different treatment. G-d forgives all crimes against Him but people might still be wronged. Hence all crimes against other people must be paid for independently of G-d's forgiveness.
(And in fact Jewish law states that all debts to other people must be paid before Yom Kippur as on Yom Kippur G-d is asked to forgive all crimes against Him and the three judges only grant atonement to those who have righted all wrongs they committed against other people.)
We do not have to understand crimes against G-d. I don't understand why homosexuality is a crime and I don't have to because I cannot possibly be the wronged party. I do not condemn others for what I don't understand.
G-d will sort it out. The only job we have is to show love and respect for everyone.
It should be legal, in my opinion, to refuse service to those whose actions one doesn't condone (as opposed to refusing service to those who skin colour, gender, or ethnicity one doesn't condone). But whether it is "Christian", I cannot say.
When Jesus was dining with prostitutes, he did so presumably to show what he thought the correct behaviour was. And his followers presumably tried to act like him.
Well, they do not any more, but they still call themselves "Christians". Something is wrong.
I think Christians should, before they worry whether somebody else commits a crime against G-d, worry about whether they themselves do commit a crime against Jesus.
Can you imagine Jesus turning away a prostitute because he doesn't condone her business? I cannot, and I don't even believe in Jesus' divinity. My respect for Jesus is based entirely on his words and actions and not on a special connection of his with G-d.
Perhaps Christians respect Jesus for his supposed special relationship with G-d and that is why they so often forget his teachings?
IDEALLY (that is, in a world where Christianity would be the ideal), the Christian shop owners would service the Lesbian couple even when nobody else does. That would be a shining example of the teachings of Jesus and is exactly the type of thing the historical Jesus did in Israel.
I regard as the most important Biblical law the law against idolatry. I believe that it is physically wrong to follow or pray to an idol and that doing so will always inevitably be part of the cause of something bad. I believe that the example here shows that idolising Jesus and thinking of him as a manifestation of G-d makes people forget his teachings.
Perhaps Jesus is G-d. But if he is, he should have known that the realisation of that fact might make people forget his teachings. I fail to see how G-d would not have known that.
As for the legal situation: if it is illegal not to serve Lesbian couples, the shop owners should pay whatever the law says. It is a duty of every believer, including Christians, to follow the law whether they like it or not. There is a right and duty to resist tyranny of course but being forced to serve customers one doesn't like is hardly tyranny.
As for the law: I think it is ridiculous that refusing service (which should be a right) is illegal and that the fine is so high. It's utterly ridiculous.