I have written several times about D&C 63. What the section is really getting at is relatively straightforward, but still hard for us to see, I think mostly because of our own traditions.
The main point is simple and perfect. It is this:
If a man who belongs to the church is doing something that would cause him to suffer in the lake of fire and brimstone, then he must either repent, or the church must cut him off.
It is simple. When put that way, it is also rather obvious. You can't be doing things that will land you in the lake of fire and brimstone and remain in the church unless you repent, cease, and desist. That is the Lord's standard.
This clearly includes a man doing any of the things the Lord openly states he will give someone a part in the lake of fire and brimstone.
We try and be clever and wiggle our way out of such a command. For instance, the Lord many times says that liars will have their place in the lake of fire and brimstone.
Interestingly, there are those among us who would argue against such a standard simply by being silly about it. For example, let us say I state such a standard and then quote an accompanying verse, such as:
D&C 63:17 Wherefore, I, the Lord, have said that the fearful, and the unbelieving, and all liars, and whosoever loveth and maketh a lie, and the whoremonger, and the sorcerer, shall have their part in that lake which burneth with fire and brimstone, which is the second death.
Then somebody will immediately loudly object saying "Are you saying people should be excommunicated if they just tell one lie!?" And, no, of course I am not. And we should have that much common sense. Because frankly I don't think any of us read that verse and think to ourselves that the Lord is saying that anyone who tells one lie will burn in the lake of fire and brimstone.
But if you do not even qualify as an "honorable man of the earth" like those who will inherit the terrestrial kingdom, if you are such a liar that you cannot even be counted as an "honorable man of the earth", then you also do not belong in the church and you should repent, or be cast out. If you are doing something that makes you merit the lake of fire and brimstone, you should repent, or be cast out.
So I will repeat the principle, and hope we can figure it out. If you are doing something for which God makes men suffer in the lake of fire and brimstone you should repent, or be cast out. And that is the principle of D&C 63. Because the church MUST, above all things, be justified before its God. It is profitable that one man should perish, rather that a whole church dwindle and perish in unbelief. Or even better, that man could just repent, given the plain options standing before him.