At least in a divorce you tend to split things (I'll keep the bed and you can have the dresser). In this case it's someone walking out the door but saying "hey, could you let me still come by for dinner?"
> May's deal permanently blocks the UK from ever fully leaving the EU without the EU's permission
That's the big joke of Brexit. Boris Johnson fantasizing about dozens of trade deals ready to be signed when the UK exits. How many are there now? Zero.
Because the UK isn't in a position as strong as Brexiteers fantasized. Ostensibly they are all for negotiating deals in the national interest, but when the other side has interests, too, that's somehow unfair and they should "go whistle".
Brexiteers are looking for handouts, not real negotiations.
My point was that you cannot invite real interest-driven negotiations and then complain about red lines the other side draws in the sand.
It's either a real negotiation or an agreement between friends.
The latter would have been preferable, but all the hatred towards the EU side made that an immediate non-starter.
But it is possible for someone to inform you that if you do take a given action, they will take draconian, but completely legal, measures against you.
> May's deal permanently blocks the UK from ever fully leaving the EU without the EU's permission
The UK, as a sovereign nation, is party to international agreements which it has signed in the past which commit it to doing various things, many of them intended to be permanent and many unrelated to the EU. One of those is with the Republic of Ireland, over peaceful relations in the UK territory of Northern Ireland. The Irish government and many Northern Irish people regard this agreement, broadly construed, as implying that no new barriers will be set up on the Irish border. Another international agreement, WTO rules, which the EU is signed up to (and the UK also intends to trade under) implies that if the UK and EU adopt different customs regimes they will also be required to actively monitor customs borders between the UK trading bloc and the EU trading bloc. The Irish government (and by an extension EU negotiators representing their interests) regards feasible customs border check regimes as violating the spirit of the Good Friday agreement over security barriers. Therefore unless and until some border solution in line with WTO regulations and acceptable to the Irish government or an adjudicating party is found (or the Republic of Ireland also chooses to leave the EU, which is highly unlikely) Northern Ireland must not become part of a different customs area from the Republic of Ireland and therefore must remain subject to some relevant EU laws.
Being a sovereign country means the possibility of reneging upon prior commitments, but it doesn't mean these agreements don't exist or that parties won't insist you honour [their interpretation of] them when offering new agreements
The "backstop" stipulates that any future trading relationship does not violate terms of the Irish peace agreement known as the Good Friday Agreement - specifically that there would be no physical checks at the border in Ireland.
Currently the border is virtually invisible because the UK and Ireland are EU members and in the same customs zone.
If, in the future, the UK decides it would prefer to leave the Customs Union (you can be in the customs union but not in the EU), then this would require a special arrangement for Ireland - probably requiring customs checking to be done between Northern Ireland and Britain instead of between north and south in Ireland.
For some people (and I don't know why - see my comment above), this is absolutely objectionable even though this arrangement would enjoy popular support in both parts of Ireland. If you're not prepared to accept this arrangement, then the UK would be stuck in the EU's customs union.
Of course if you want to leave the customs union (otherwise what's the point of brexit), then you have to accept the special arrangement for Northern Ireland or basically renege on the terms of Irish peace agreement.
Sure, customs checks at ferry and airports to the UK mainland would entail considerably fewer practical delays, smuggling opportunities and security issues than customs checks between a village and its nearest supermarket on the Irish border, but unionists can legitimately argue an Irish Sea border violates their understanding of the spirit of the Good Friday Agreement. Though it'd also be perfectly fair to ask their politicians who all backed Brexit what did you expect to happen?
They can probably still leave as a sovereign entity, in the same way I can technically ignore my loans that I initially agreed to pay back. There will be consequences.