Researchers found before walking down the aisle, couples can expect to have sex more than four times a week.
But after three years of life as man and wife most couples are lucky to have sex just once every seven days.
Astonishingly, just under half of all married people say their relationship with their partner is more like friends than lovers.
Extra-marital dating service www.lovinglinks.co.uk conducted the poll of 3,000 married people.
"Unfortunately, while you can be deeply in love with someone and want to spend the rest of your life with them, it is also possible to want more from the relationship," the Daily Mail quoted a spokesman of the service as saying.
"A partner might be supportive, funny, intelligent, and kind, but if they don't inspire confidence in the bedroom, or don't meet expectations sexually life can be frustrating.
"And it is at times like this when eyes start to wander, and folks start to think about having a no-strings affair with someone else.
"After 14 years of providing a service where married people can look for sex elsewhere, we have good reason to believe many relationships are strengthened by a little out-of-marriage activity," added the spokesman.
The poll shows 59 per cent of couples believe their sex life has worsened since marriage because they no longer make an effort with each other any more.
And a third no longer fancy their partner as much as they did in the early days, with 43 per cent claiming their loved one has let themselves go.
Unfortunately, eight in ten couples are in a bit of a sexual rut - having sex at the same time, in the same place and in the same positions every time they sleep together.
In fact, 79 per cent of people are happier getting a good night's sleep than making the effort to have spontaneous sex in the middle of the night.
But when it comes to having an affair, two thirds of those who have dabbled admit the sex was mind-blowing compared to the once-a-week missionary with their husband or wife.
And even a fifth of those who haven't yet played away from home would have a one-night stand if the opportunity presented itself or if their sex life with their partner doesn't improve.
The same percentage even said they would understand if their partner suddenly confessed to sleeping with someone else.
Unsurprisingly, a quarter of those polled admit to having had a one night stand to satisfy their craving for good sex.
And 14 per cent have even embarked on a hot blooded affair.
'Modern marriages are becoming a little more open where sex is concerned, and these days we are quicker to forgive if someone has a little one night stand," said the Loving Links spokesman.
"It is never nice to learn that our partners aren't satisfied in the bedroom department, but if sleeping with someone else spurs couples on to make more of an effort with each other that can only be a good thing," he added.
Two thirds of couples blame their hectic lifestyle for their terrible sex life, and 80 per cent are often too tired to bother once the day is over.
Seven in ten people reckon they might be inclined to make love more if their partner made more of an effort romantically.