But, as you so abundantly show in your post, the supposed "significant difference" between the immoral and the illegal is shifting rapidly! If today you can dismiss sodomy and incest as being in the realm of "immoral but not illegal", then what, exactly, is the basis for declaring bank robbery illegal? Why, inherently, is stealing illegal? If it is not because our society has declared it to be wrong, what is the reason? What is the basis for laws at all?
You might say something should be illegal if, and only if, it harms somebody else. While there is plenty of debate to be had over that idea in the first place, I would press further and ask, Why? Why is that the standard for individual liberty? Why should it be illegal for me to harm someone else? What makes that standard more desirable than a biblical standard, or a simple majority rule standard?
See, the slippery slope is this: Once enough people deny any difference between right and wrong in their personal lives, then the laws created by and for those same people will inevitably follow the same path, albeit in something of a delayed fashion. If there is no good and evil in our personal, private lives, there is necessarily a similar attitude toward our civic attitudes and actions. Those who think they can completely separate their personal and public lives are deceiving themselves.
Are you seriously proposing that everything you personally believe to be immoral should be illegal -- with men with guns to enforce it? That is a conclusion that can be drawn from "no separation between public and private lives". For example:
If you belive people should go to church on Sunday -- then should everyone who doesn't be placed in Stocks?? And businesses which open on Sunday should be confiscated??
If you belive that people should only worship in a Protestant Church -- or a particular sect of a Protestant Church -- then should all Catholic Churches, and all other churches of all other Protestant sects, be confiscated or burned?
Europe had about 400 years of that kind of thinking, with more than enough death and destruction to go round, and they finally declared an exhausted truce, and saw the benefits of "tolerance" for other religions. Islam has not yet seen this light, and is due for a lot of death and destruction until they do.
I never even implied that there was no difference between good and evil. I strongly suggested that there was a difference between what you require of yourself, and what you should be willing to require of others by force of law. And once you have agreed that there is a difference, you should start to think about how you should decide; then you can persuade others to decide using the same principles.