Is the Catholic position a "take it or leave it" kind?

[Aractus]

Or rather one should switch to a more generic Roman Catholic church? Or even to "bare" Christianity? For consequence sake?

Excommunication is indeed biblical (eg 1 Corinthians 5:1-13), but yet a lot of Christians actually look down upon those who practice it.

And as a follow up - what is the reasoning behind ability to make an exception and not realizing that's a normative claim from outside of religion? Like, "if I know how to filter the Old Testament's bad ethical rules, I cannot be certain of the rest of them, and/or it is I who know something about Ethics, not the Old Testament". What stops people of faith from realizing this? How do they explain this to themselves?

They can't. They often claim there's a difference between ceremonial and moral law - interestingly keeping the Sabbath is clearly defined as a moral obligation in the OT - but then most Christians throw out most of the moral law with the ceremonial law. Given that Jesus taught that divorce is not lawful, there can be very little doubt that these changes were indeed made by the first and second century church and not taught by Jesus or his 12 disciples.