Let us begin by picturing a couple in a cafe at eleven o’clock on a Saturday night in a large city, eating ice cream after seeing a film together.

There is doubtless a biological explanation for the sexual excitement this couple are feeling, connected to an unconscious narrative about reproduction and genetics, but the man and the woman are also turned on by the overcoming of the many barriers to intimacy that exist in normal life — and it is this dimension we can focus on to explain the greater part of the eroticism they will experience on their way to the bedroom.

Spoon in hand, the woman is describing a holiday she recently took to Spain with her sister. In Barcelona, she says, they visited a pavilion designed by Mies van der Rohe and ate in a restaurant that specialised in seafood with a Moroccan influence. The man can feel her leg beside his, and more specifically the elasticity of her black tights as they taper to the hem of her grey and yellow skirt. When she is in the midst of relating an anecdote about Gaudi, he moves his face towards hers, ready to pull back if she gives any indication of fear or disgust — but his advance is met, to his enchantment, by only a tender and welcoming smile. The woman shuts her eyes, and both parties register the unique, unexpected combination of moisture and skin across their lips.

The pleasure of the moment can be understood only by considering its wider context: the overwhelming indifference against which any kiss is set. It goes almost without saying that the majority of people we encounter will be not merely uninterested in having sex with us but positively revolted by the idea. We have no choice but to keep a minimum of sixty or, even better, ninety centimetres’ distance between us and them at all times, to make it absolutely clear that our compromised selves have no intention of intruding into their personal spheres.

Then comes the kiss.

The deeply private realm of the mouth — that dark, moist cavity that no one else but our dentist usually enters, where our tongue reigns supreme over a microcosm as silent and unknown as the belly of a whale — now prepares to open itself up to another. The tongue, which has had no expectation of ever meeting a compatriot, gingerly approaches a fellow member of its species, advancing with something of the reserve and curiosity exhibited by a South Sea Islander in greeting the arrival of the first European adventurer. Indentations and plateaus in the inner lining of the cheeks, hitherto thought of as solely personal, are revealed as having counterparts. The tongues engage each other in a tentative dance. One person can lick the other’s teeth as if they were his or her own.

It could sound disgusting — and that’s the point. Nothing is erotic that isn’t also, with the wrong person, revolting, which is precisely what makes erotic moments so intense: at the precise juncture where disgust could be at its height, we find only welcome and permission. The privileged nature of the union between two people is sealed by an act that, with someone else, would have horrified them both.

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The above excerpts are taken from Alain de Botton’s book How to Think More About Sex.