In sky Atlantic's glossy new thriller, The Affair, life appears close to perfect for Noah, a ruggedly handsome schoolteacher who lives in New York with his college sweetheart and three children.

Noah, played by British actor Dominic West, puts all this seeming contentment at risk when he embarks on an affair with Alison (Ruth Wilson), a pretty, married waitress, and we watch as the psychological effects of their actions unfold in scenes of breathtaking tension.

A good part of the drama's intrigue stems from the way events are portrayed from both Alison and Noah's perspectives.

The Affair is compelling entertainment, yes, but it raises moral questions that resonate with many. Why did they do it? Was it just about sex? Did they feel guilty? Which party, if either, was more in the wrong? Every day in Britain tens of thousands of people cheat on their partners, many now seizing the opportunities thrown up by the internet.

Illicit Encounters, the UK's largest dating site for married people, has more than 900000 members on its books. Here, two members who met through the website tell their stories.

l Melissa is divorced with children. She lives in Surrey and works part-time as a teacher. She started looking for a new partner online five years ago and began an affair with Seb.

"I had been married for 30 years when I discovered that my husband had been cheating on me for a long time. I was completely traumatised. One of the dating sites he'd been using was still open on his computer and that's where I got the idea for joining a website for married people.

"Seb wasn't wildly sexy and all the rest of it - but we just clicked.

"On our third date, we met for dinner at an upmarket hotel where Seb had booked a room for the night.

"I was incredibly nervous. I'd never been with anybody other than my husband.

"The sex was a revelation. Sometimes I ask myself how I could be doing to Seb's wife what my husband did to me. But I feel that if he wasn't with me, Seb would be with somebody else.

"I've been seeing Seb for five years now. We talk twice a day and try to meet up once a week. For me, the benefits of our unusual relationship are enormous.

"We have great sex and a lovely friendship.

"Falling in love with Seb is out of the question. Seb was absolutely clear from the start that he was not going to leave his wife, ever."

l Seb is married with children and grandchildren. He lives in Wiltshire, works part-time as an engineer.

"One day my very lovely wife, who I've now been married to for 49 years said, quite casually, that she didn't want any more of 'that sort of thing'. My life fell apart. That was eight years ago. Eventually I decided I'd better do something about it.

"I looked on a few dating websites and thought Melissa's profile was very amusing. She wasn't drop-dead gorgeous but I thought her photo looked all right. I wrote to her that I liked what I saw and wondered if we could talk. Two weeks later we met up for tea at a hotel.

"My first impression of Melissa was of a tall, elegant, interesting woman. She wasn't in the first flush of youth but I felt an immediate interest. And we had lots to talk about. We met up a couple more times then arranged to have dinner at a country hotel.

"We speak twice a day. We see each other once a week and usually spend 24 hours together.

"The sex is absolutely wonderful and without that our affair wouldn't have lasted five years.

"I worry about being caught, but I don't feel especially guilty because I'm not taking from my lovely wife anything she desires." - The Daily Telegraph