The whole point of Brexit is that we should trade with a group of countries with no political interference from either side so that no-one should interfere in a country's own affairs. The trade agreement should be limited to the countries involved and not affect the way any of the parties trades with other countries.
Is that outlandish? It's the way that most countries deal with each other, I think that a sensible person would say, "No." However, many Remainers cannot imagine that they should let it happen without claiming that their whole lives and the lives of their children have been ruined forever and trying to subvert it. They should ask themselves, "Why?"