With Valentine's Day once more upon us, the sensible beer drinker is switching to stout or porter, known the world over as aphrodisiacs
"At the breast ... that's where I first tasted this beer." The dynamic young German brewer was telling me about his spicy-tasting black beer from Thuringia.

The blackest beers of all are porter and stout (the latter usually, but not always, bigger-bodied). Not quite mother's milk, perhaps, but lactose is used in several sweet stouts, sometimes imparting a slightly yoghurty flavour. Lacto Stout, made by the local brewery on the island of Malta, adds vitamin B, too. Its label shows not Venus, but Neptune, king of the sea. We are luxuriating in deep waters here.

The "decadent" writer JK Huysmans, in his novel, Against Nature, described a quest for the rare and perverse in sensation. He imagined an all-black meal, involving Russian rye bread, caviar, game "in sauces the colour of boot-polish", plum puddings, porter and stout. Catherine the Great, who was rumoured to have had sex with her horse, commanded that porter be brewed in St Petersburg, and it still is.

In Finland, I have sweated naked in the sauna, jumped in a muddy lake, then consumed the oily Koff Porter with smoked eggs. Unfortunately, all my naked companions were male.

For years, Swedes required a doctor's prescription to buy their creamy Carnegie Porter. I sank a few with oysters not long ago at a restaurant called the Wasahof in Stockholm. As the double aphrodisiac began to take effect, I started to lust after the only lone woman at the table, until I learned that not only was she happily married, but to the son of former world heavyweight boxing champion, Ingemar Johansson.

In its Georgian and Victorian heyday, porter was offered in London pubs with bar-snacks of free oysters. These were inexpensive and plentiful in the days before the Thames Estuary and the North Sea were polluted. Such delights are still on offer, at a price, at Sweetings, in the City of London. For customers who are insufficiently stout-hearted, this lunch restaurant offers the black stuff diluted with Champagne, by the pint, in a silver tankard. This can, in my experience, lead to afternoons away from the office.

A stout designed especially to accompany oysters was launched in recent years by Marston's, one of the world's most distinctive breweries and currently under threat of takeover. This dry, oaky-tasting brew, at a modest 4.5 per cent alcohol, has blown away far stronger brews in blindfold tastings. I was amused to see it being recommended in one of the laddish magazines recently. Are they finally learning that there is life beyond lager?

The Isle of Man has a tradition of producing stouts that contain oysters, boiled in the brew-kettle. This was revived in recent years by the local micro-brewery. Bushy's Oyster Stout has a salty, gamey note. For St Valentine's Day, I may have to head for Dublin. The Porter House in Parliament Street also boils oysters in the kettle. Its oyster stout is salty, peaty and sweetish.

In Jamaica, a classic slogan informed tired lovers that "Dragon Stout Puts It Back". I find Dragon a little on the sweet side; there seems to be more fruitiness in Prestige Stout, from Haiti. In New Orleans, a beer called Blackened Voodoo turns out to be a faintly smoky dark lager, and goes very well with Oysters Rockefeller. In Delaware, the scary-sounding Dogfish Head brewery has a rooty stout flavoured with chicory, just the thing for sensuous crab-feasts.

In Michigan, the picaresque town of Kalamazoo (where Glenn Miller had a girl - or was it Tex Beneke?) has a brewery making no fewer than half- a-dozen porter or stout variations. Proprietor and former disc-jockey "Doctor" Larry Bell brews a herbal-tasting one with locust pods (roasted to ensure that they are no longer poisonous). The favoured accompaniment is pickled hot peppers with quail's eggs; there's decadence for you.

In San Francisco's Stanford Court Hotel, I first entertained the woman with whom I have since shared my life. We had a meal that began with prawns marinated in smoked beer and served with Guinness, and ended with apple bread pudding and Samuel Smith's Imperial Stout.

Had we been Chinese, we would have waited until the Eighth Moon ... and bathed our newborn baby in Guinness's extra-strong Special Export Stout. It is all, I am assured, a question of yin and yang.