Aug. 19, 2009 -- It's an age-old question every couple faces: Can men and women really be "just friends" or does sexual attraction and jealousy take over? What if one person stays in touch with an ex, or has an opposite-sex pal?
"Good Morning America" special contributor Steve Harvey agrees with Billy Crystal's character in the movie "When Harry Met Sally," who famously said, "Men and women can't be friends ... the sex part always gets in the way."
A study in the Journal of Personality and Social Psychology estimated that an opposite-sex friendship can result in an affair as often as 15 percent of the time.
Who better to weigh in on this classic relationship question than the man whose new relationship book "Act Like A Lady, Think Like A Man" sits at number one on the New York Times Hardcover Advice bestseller list?
Author, comedian, actor and host Steve Harvey tackled this topic with a number of couples.
Justin Vanlandschoot and Stacy Marsch
Married for four years
Vanlandschoot, 33, had a female friend who was coaching him in a speaking competition. Marsch, 37, knew about her, but one day saw an e-mail from her husband to this woman and it was signed, "Love ya."
Although Marsch says that men and women can have opposite-sex friends, she felt uncomfortable and confronted him.
Vanlandschoot says although there was absolutely nothing going on between him and his friend, it was not worth jeopardizing his marriage and he ended the friendship.
"You typed, 'love you'?" Harvey asked Valandschott. "That's pretty dangerous ... I can't even type 'I like you a whole lot' without starting a major fire."
He told Valandschott that he did the right thing by ending the friendship.
"I'm just of the belief that a couple should form a two-handed circle," Harvey said. "Outside relationships ... what good do they bring to your marriage?"
Aaron Bouw and Lindsey Dahlin
Dating for three or four weeks
Dahlin, 25, says it is fine for men and women to "just be friends." However, Bouw, 29, says that women pull the naive card" and don't realize it when a male friend is interested in them. Bouw says he trusts Dahlin, he just doesn't trust her guy friends.
"I know how men think," Bouw said. "We have one thing on our mind."
Harvey told Buow he was trying to mark his territory, not protect Dahlin.
"That's how men are, that's how we really think," Harvey said. "It's almost virtually impossible for a man to be a woman's friend. The only reason we're your friend [is because] you've made it perfectly clear it will go no further than this."
Can Men and Women Just Be Friends?
Sharon and Bill Brewser
Sharon Brewster, 53, said she almost called off her wedding because she found a text message from another woman on her husband's cell phone that ended with "sweet dreams."
"I heard Bill's phone go off," she said. "I went out and confronted him. I was very upset."
"She's certainly not saying have sweet dreams about your wife," Harvey said.
Sharon's husband, Bill Brewster, 52, told her that he always got along better with women than men. But knowing that Sharon does not believe men and women can be "just friends," he did not tell her about some of his female friends.
"The big mistake that I made was that she didn't know about her," Bill said.
Mary Fitzgerald and Bill Soldwisch
Dating for 11 years
Fitzgerald and Soldwisch both have friends of the opposite-sex and say it has never been a problem. Although the couple has not married, they remain committed and say they would never do anything to jeopardize the relationship.
Having female friends has "never been a problem," Soldwisch said. "They've been my best friends ... I've been fortunate to involve myself with confident women in my life."
"They'll always be temptation, but when you're in a true committed relationship, why ruin a great thing?" Fitzgerald said.
Harvey was surprised Fitzgerald didn't want to get married.
"I have no particular interest in being married," she said. "We don't need the piece of paper."
They do refer to each other as husband and wife, to make it "easier for the general public," Soldwisch said.
"I just don't know a woman who doesn't want to be married," Harvey said. "I'm stuck."
Can Men and Women Be Friends? Viewers Weigh In
"GMA" also received hundreds of e-mails from viewers eager to join the conversation.
Angela McDaniel from Lufkin, Tex., wrote in about emotional infidelity.
"My husband has had several female friends that I didn't know about. If your spouse doesn't know about his friend then you are having an emotional affair! If you weren't there would be no reason to sneak & talk to the friend."
Brenda Velasquez from Modesto, Calif., says men and women can be friends without causing problems in their marriage. "Take me for an example. After my first marriage, we're still friends. In fact my ex-husband is renting a room from my current husband and me. In my opinion, it is just about how civilized and grown up you are in the situation and relationship."
Heather from Birch Run, Mich., e-mailed "GMA" to ask Harvey a question.
"My best friend is a male. We dated for a very short time several years ago right after high school. When my now husband and I started dating he knew about him and we hung out several times. But about two years after we got married my husband started making comments about him. Now every time we fight, he throws him up in my face. I have no clue how to get him to stop. Any ideas?"