You've just been through the longest breakup on Earth, and are finally finished with your ex. 

Well, almost finished. Except for the sex. 

While some couples swiftly sever ties when the relationship ends, many succumb, again and again, to the familiar safety of their old flame. So what is it that drives people back into each other's arms long after the relationship has ended? 

For some, it's a comfort thing. For others, it's about residual emotions and the need to feel loved. And then there are those who do it just because there's no one else to keep them warm on those cold, lonely nights. 

"I don't think it's uncommon for people who have broken [up] to get back together occasionally for one more fling -- especially at times of low self-esteem," said Dr. J. Michael Faragher, an expert in addictive behaviors and sexuality at the Metropolitan State College of Denver. "Sometimes it really feels good to go back into the arms of a person you cared about before." 

Been Down That Road

"Tara" Miller (not her real name), a 24-year-old legal research assistant, said she has gone back to her two most significant exes for sex, primarily because she can't feel intimate with someone she doesn't know well. 

"It's a way to have sex without dealing with the emotional consequences of casual sex," Miller said. "I have a hard time having sex with someone I'm not in a relationship with." 

Miller broke up with her boyfriend from high school when they both went to college. But they had sporadic sex for two years. She said she wasn't comfortable with the less-than-monogamous hook-up scene most of her friends were involved in. 

The reasons for continuing the sexual relationship with her second ex-boyfriend, with whom she split permanently a few months ago, are quite different. 

"That's the way I resolve feeling bad about it," Miller said. "I can't completely let it go yet. We have a very, very good relationship and we're wonderful friends." 

Faragher said the emotional and health-related risks people take when venturing into new intimate relationships make it tempting not to bother with the unknown at all. 

"There are multiple factors that make this behavior understandable," he said of the sex-with-ex phenomenon. "The big ones are fear of disease and the difficulty in starting a new relationship. Dating is riskier now. It's a pretty good incentive to maintain the sexual part of a relationship with someone you believe is safe." 

And the sexual connection between former lovers often stays intact after everything else has crumbled. Even if some aspects of a relationship were destructive, "it doesn't necessarily mean that sexually you weren't still a good match," Faragher pointed out. 

Jenny, 24, a marketing coordinator for a cosmetic company, said she and her ex-boyfriend dated for seven years, beginning when they were 13 years old. They continued having sex off-and-on for three years after the split -- even though both dated other people. 

"We were so young and kind of grew into our sexuality together," said Jenny, who asked her last name be withheld. "It was always a temptation to go back to him. He knew me, and we were so familiar with each other. It always seemed natural." 

Sex, she said, was never as good with the second boyfriend as it had been with her ex -- in large part because she felt she couldn't completely be herself with her new beau. 

"When you start dating somebody new, it's like you're testing the water," Jenny said. "You're still tiptoeing around. You really haven't gotten to that point where you're so comfortable with each other that you're not inhibited about doing or saying certain things." 

The Dark Days 

The desire to romp with an ex often strikes during those lonely times, when the need to feel cared for by someone -- anyone -- is particularly strong. 

"Sometimes it's nice just to feel like someone loves you, temporarily, because you know the person always will," Miller said. 

Continuing physical intimacy can also be comforting because it reinforces the fact that an ex still has feelings about you. 

"It's about more than just sex," Faragher said. "It's going back home for one more pat on the head -- a reassurance that the other person still cares about them in some way." 

But it can sometimes prevent a person from moving on. Miller said there has to be a mutual, if unspoken, agreement both people aren't doing it with any hidden agenda. 

"If you're trying to win him back, you're playing the wrong game," she said.