Caroline Sommerfeld is very right wing. Helmut Lethen is very left wing. Both are German academic writers. She regards him as fixated on hating Nazis. He regards her as a racist bigot. They are in the news because they love one another and are happily married.

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We are all fine with marriages across ages, classes, races, genders and religions. They prove we are a tolerant and well-bonded society. Politics seems different. As it increasingly polarises right and left, it is leaving behind a bleak no man’s land that, even in our broad-mindedness, we cannot bridge. How can you love someone whose politics you despise? Surely you should hate, or at least avoid them.

Lethen was a child of 1968, when German youth began to discover the truth about Hitler. He joined a Maoist group and is an expert on the Nazi elite. Sommerfeld is a generation younger and objects to his “pathological obsession with the Holocaust”. She is a member of the new right and has written a book about how to deal with liberals. She opposed Syrian immigrants, would not want a black neighbour and regards Islam as an existential threat to Germany. Lethen finds her views abhorrent. We are not told how often they throw coffee at each other. They have three children.

I find the Sommerfeld-Lethen story strangely reassuring. Dating algorithms would have this couple at arm’s length from each other. If it was Tinder you would sue. Social media would feed them widely divergent opinion and news. The theory of assortative mating holds that we are ever more averse to partners unlike ourselves. It speaks well of a German university that under its aegis two such people could meet “live”.

The flight from the political centre ground has become alarming, not because it neglects certain points of view but because it prevents arguing over them. Divergence is not debate. The grievance factory that is identity politics excuses “why I don’t have to talk to you”. It emphasises group differences, not shared values. The latest hysteria about no-platforming and safe spaces may be overdone. But it is censorship that shows an authoritarian fear of argument. It says we should all retreat to our castles and speak only in our own language.

We must talk across the divides of politics, as we try to do across those of sex, religion and race

The past two years have been traumatic for many who thought themselves politically broad-minded. After Donald Trump’s election, what most shocked the obituarist of American liberalism, Mark Lilla, was that “I no longer know my own country”. Metropolitan America had grown so detached from its hinterland, it no longer recognised it. Many Britons had the same experience over Brexit. I know friendships that have not survived.

John Stuart Mill said, “He who knows only his side of the case knows little of that”. When the political psychologist Jonathan Haidt tried to puzzle out “why good people disagree”, he found himself digging far below “reasons” into hidden insecurities and prejudices. His book The Righteous Mind explores how reason becomes subordinated to a “groupish righteousness”. We do not really want friends and loved ones to argue with us. We want them to echo our views, as a proxy for affection.

When I first read about Sommerfeld-Lethen, I assumed they did what most politically mixed marriages did, and kept politics out of the bedroom – if not out of the family altogether. Apparently they argue all the time. They have some no-go areas, such as whether Stalin’s holocaust was worse than Hitler’s. But they conduct argument “on an assumption of goodwill and rationality”. They obey the oldest rule of politics, that of courtesy.

This makes me realise how hard it is for most people to handle a heavy political argument without lurching into a row. It is so hard we try to avoid even associating with people likely to prove disagreeable. Yet voting is not the only debt we pay to democracy. I once found myself trapped for an hour next to a member of the National Front, who clearly revelled in lecturing me. I found it unpleasant, but by the end I admit I understood him well enough to counter at least some of his points. He was not of my tribe, but I realised we must get beyond tribe if we are to keep the democratic conversation flowing. I was brought a little closer to the liberals’ greatest (and most arrogant) conundrum: why so many people fail to agree with them.

Politics starts with conversation. We must talk across the divides of politics, as we try to do across those of sex, religion and race. Otherwise, as Mill said, we will not really understand ourselves. We will end up shouting in the dark. A good resolution would be always to have one friend at whom we regularly want to throw a cup of coffee – with love.

• Simon Jenkins is a Guardian columnist