AdultFriendFinder, which is owned by Penthouse magazine publisher Penthouse Media, allows members to list themselves as "attached" or "married" -- as well as "single," "divorced" and "widowed" -- on their profiles, an option implemented after users suggested it, said Penthouse Media Chief Executive Marc Bell.
But the site doesn't encourage infidelity, Bell told ABC News, any more than sites like Facebook and MySpace do.
"It's just another social networking site," Bell said.
The same can't be said for AshleyMadison.com, the online dating service run by Noel Biderman. The Toronto-based agency makes no bones about the service it provides. In fact, the slogan topping its home page reads "Life is short. Have an affair."
Biderman said AshleyMadison.com was founded in 2001 to meet the needs of married people who were trolling singles' Web sites in search of extramarital mates. The site, Biderman said, has seen its membership double over the past year to more than 2.2 million.
For $49, members can create profiles and send a limited number of e-mail and instant messages to potential mates. The more e-mails and messages you want to send, the more you have to pay.
Biderman said most men find success with the site's $49 package, but some choose to upgrade to pay for more messages. Because 65 percent to 70 percent of the site's users are men, Biderman said, heterosexual male users typically need to send out more messages before successfully wooing a woman.
Women, Biderman said, make just 5 percent of the purchases on the site.
"The site is definitely designed to monetize the men and validate the women in that regard," Biderman said.
While most male members have had success with just a $49 package, Biderman said he also knows members who over the years have spent thousands of dollars to have multiple affairs.
AshleyMadison.com isn't the only option for Web-savvy cheaters. PrivateAffairs.com allows infidelity-minded users to place personal ads for a monthly subscription fee of $29.95, or $109.95 for a year.
The site advertises itself as a serious place. According to the Web site, it includes"[n]o game players; only qualified people seriously seeking an extramarital experience."
Even if a relationship appears to take place only in cyberspace, Houston said it can be just a matter of time before a would-be cheater turns his online hookup into a real one.
"If you're not changing partners, if you're not corresponding with different ones, it's always with the same person, sooner or later, you're going to want to meet that person," she said. "That's when it really begins to take a downward slide."
When things do get physical, the amount an adulterer shells out, she said, can vary depending on the type of relationship. While a one-night stand's expenses may be limited to just dinner or a hotel stay, she said, someone involved in a long-term affair might make purchases with the intention of improving their own appearance.
A cheater might spend money on a new wardrobe or a gym membership to impress his or her new mate, Houston said.
"He's gone overnight from Kmart to Armani -- there's something wrong with that," Houston said.
Biderman said that when it comes to affairs, there are also those who pinch pennies.