Researchers found that before marriage, couples can hope to have sex more than four times a week. But after three years of married life, there is a dramatic drop in their sex life and most couples have sex just once every seven days.
The survey was conducted among 3,000 married couple.
Six out of ten couples think that marriage has completely ruined the excitement of having sex, Daily Mail reported.
Another astounding result of the survey was that just below half of all married couples said that their relationship was more like friends than lovers.
"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," a spokesman for extra-marital dating service www.lovinglinks.co.uk was quoted 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," he added.
He said: "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...We have good reason to believe many relationships are strengthened by a little out-of-marriage activity."
The survey revealed that 59 percent of couples think that their sex life had worsened after marriage.
Incidentally, eight in ten couples were in a sexual rut by having sex at the same time, in the same place and in the same positions every time they slept together.
As a matter of fact, 79 percent of the respondents were happier getting a good night's sleep than making the effort to have spontaneous sex in the middle of the night.
Two thirds of the couples who had an affair admitted that sex was mind-blowing compared to the once-a-week sex with their husband or wife.
A fifth were ready to have a one-night stand if the opportunity presented itself or if their sex life with their partner didn't improve. And nearly a quarter said they had a one-night stand to satisfy their craving for good sex.
The Loving Links spokesman said: "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."
The results of the survey showed that almost two thirds of the respondents blamed their hectic lifestyle for their unhappy sex life and 80 percent were often too tired to bother once the day is over.
