They are allowed to sign up for free as a way of ensuring the numbers are balanced between the sexes.I register, and enter the murky world of two-timing technology, taking note of the warning on the site: "Not all affairs have a positive effect on a marriage." What a masterpiece of understatement.
Postings such as: "I want a man who can look after me and knows how to treat a woman. I'm surprised and unsettled by the forward tone of some of the material. Determined to avoid the connotations, I reply: "The Beatles." I never hear from her again.
But it claims to have more than 100,000 members in the UK.
Many of them are middle-class, many have young children.
Suddenly the realisation of how odd it is to meet a stranger with the express intention of having an affair dawns on me. It's more like Alan Sugar interviewing an apprentice.
But she is an old hand at this type of encounter and tells me she's met many men through the site, and that I was probably the only one who hadn't lied about my age.
"Sophia" tells me she thinks relationships have a shelf life of about ten years before boredom sets in, but that she stays married to ensure her children have a stable home.