ORAN, Algeria — AFTER Tahrir came Cologne. After the square came sex. The Arab revolutions of 2011 aroused enthusiasm at first, but passions have since waned. Those movements have come to look imperfect, even ugly: For one thing, they have failed to touch ideas, culture, religion or social norms, especially the norms relating to sex. Revolution doesn’t mean modernity.

The attacks on Western women by Arab migrants in Cologne, Germany, on New Year’s Eve evoked the harassment of women in Tahrir Square itself during the heady days of the Egyptian revolution. The reminder has led people in the West to realize that one of the great miseries plaguing much of the so-called Arab world, and the Muslim world more generally, is its sick relationship with women. In some places, women are veiled, stoned and killed; at a minimum, they are blamed for sowing disorder in the ideal society. In response, some European countries have taken to producing guides of good conduct to refugees and migrants.

Sex is a complex taboo, arising, in places like Algeria, Tunisia, Syria or Yemen, out of the ambient conservatism’s patriarchal culture, the Islamists’ new, rigorist codes and the discreet puritanism of the region’s various socialisms. That makes a good combination for obstructing desire or guilt-tripping and marginalizing those who feel any. And it’s a far cry from the delicious licentiousness of the writings of the Muslim golden age, like Sheikh Nafzawi’s “The Perfumed Garden of Sensual Delight,” which tackled eroticism and the Kama Sutra without any hang-ups.

Today sex is a great paradox in many countries of the Arab world: One acts as though it doesn’t exist, and yet it determines everything that’s unspoken. Denied, it weighs on the mind by its very concealment. Although women are veiled, they are at the center of our connections, exchanges and concerns.