Seven-year itch now down to three years: Why?

(CBS) Seven-year itch? These days, couples' passion starts to cool after only three years, according to a new survey.

Weight gain by a partner, lack of money, snoring, and overexposure to the in-laws are top passion-killers, the Daily Mail reported. Did someone say lack of racy underwear? That's another biggie, along with toenail clippings left on the bathroom floor.

What explains couples' lack of tolerance?

"Longer working hours combined with money worries are clearly taking their toll on modern relationships and we are seeing an increasing trend for solo holidays and weekends away from marriages and relationships in order to revive the romantic spark," Judi James, the pollster who oversaw the survey, told Reuters.

The survey of 2,000 adults in steady relationships - commissioned by Warner Brothers to promote the release of its new comedy, "Hall Pass" - showed that couples spend less time in the bedroom as they become more annoyed with each other. Fifty-two percent of couples in new relationships reported having sex at least three times a week, as compared to 16 percent of those whose relationships were at least three years old, according to Reuters.

Couples seem to compensate by spending more time alone and taking separate vacations, according to Reuters.

What can couples do to keep the home fires burning? Recognize that love is tough, and that it's perfectly normal to think about straying as relationships evolve. That's the word from Drs. Charles and Elizabeth Schmitz, the authors of "Building a Love that Lasts" and bloggers for Psychology Today. Recurring fantasies about someone else suggest a problem that needs to be addressed head-on - sometimes with the help of a marriage counselor.

But the Schmitz's simplest observation may be the best piece of advice for couples:

"Love is tough."