Cheating on your spouse is always immoral, sometimes illegal, and if that doesn't matter, a wide range of Web sites are ready to help you play around.

Are you married and looking for a one-night stand? Need a soul mate to fill the void that's been growing since your wedding day? Even if you just need an alibi to explain where you were last night, there are companies especially designed for the married-but-looking clientele.

The ease of the Internet is one reason women are quickly catching up to men in the arena of extramarital nookie, according to Newsweek. Nowadays, an estimated 30 percent to 40 percent of wives are unfaithful, compared to 50 percent of husbands, therapists told the news magazine.

To show how fast the world is changing, only 10 percent of married women admitted to infidelity in 1991, according to a poll by the National Opinion Research Center at the University of Chicago. Ten years later, that number jumped to 15 percent for women, while the level of unfaithful men stayed a constant 22 percent.

Can we believe these numbers? Why would husbands and wives be honest with pollsters if they can't be honest with each other? What can't be denied is the growing number of Web sites catering to philandering, discreet dating and other services that may do a marriage a disservice. Here are a few:

1. Wages of Sin (and Various Payment Plans) You pay a price for cheating — and many dating sites for wed wanderers offer various payment plans that won't stick out like a sore thumb on your credit card bill. AshleyMadison.com, a dating site for married adults "with unmet needs," claims revenue has shot up 10 percent this year, now that it expanded its billing methods to accept debit cards along with credit card payments. Don't worry about leaving your spouse's divorce lawyer an electronic money trail straight to your secret lover. Your credit card will merely show a charge from "Ashley Madison," which sounds more like an accounting firm than a dating service that boasts the slogan: "When Monogamy Becomes Monotony." About 160,000 people have registered, a no-cost endeavor. You get to post a profile with a nom de plume, and, if you dare, a photo. You only pay if you want to contact other members. Newbies are encouraged to get specific about what extramarital pleasure they're seeking. "Swingers" should distinguish themselves from those seeking a "secondary relationship" — a long-term romance that's not necessarily sexual. A "tertiary relationship" is a polite way to refer to a one-night stand. And you should be warned: Some married daters expect you not to cheat on your mistress with another mistress — a concept known as "polyfidelity." "Our service is not meant to glorify or promote infidelity," says operations director Darren Morgenstern, who married shortly before Ashley Madison opened three years ago, in a press release. "We're simply offering a safe and anonymous way for people to communicate with each other once they've made up their mind to explore options outside their relationship."

2. Honor Among Philanderers? A Lothario's Creed From one philanderer to another, are you emotionally prepared for an affair? Can you handle the guilt, hide incriminating receipts and delete computer files that would spell ruin in divorce court? "Unless you are the only one who has access to your computer, don't bookmark this webpage," visitors of philanderers.com are warned. "The contents can bury you!!!" This warning comes from a man who identifies himself as Doug Mitchell. He won't give out his real name because, in addition to a wife, two children and a dog, he has had a girlfriend for seven years — just about as long as he's been running this site. "I thought I was alone when I started this site," says Mitchell, who describes himself as a 40ish importer-exporter from Canada. "I couldn't find anywhere on the Internet to turn for advice." Mitchell says he's still dating the same woman and that his marriage has actually improved because he's found a way of life that suits him. "It's not for everyone. You have to be prepared," he says. "My girlfriend knows I run the site. My wife does not." Would-be philanderers should be warned of the Web site's disclaimer against any liability, should your spouse get wise and take you for all you're worth. You are also warned that breaking your marriage vows is against the law in some jurisdictions. If you're still bent on cheating, however, you'll get free how-to guides and handy — presumably tested — advice.

Never use credit cards, a hotel phone or let anyone take a picture. Toothpaste is apparently great to remove a lipstick stain. If you're still worried about telltale signs of a lover on your apparel, stop at a gas station, smear yourself with motor oil and claim you slipped while pumping gas. Better to ruin a shirt than a marriage. Condoms are part of the philanderer's code, Mitchell says. And it's a good habit to use generic nicknames like "honey" and "dear" to avoid mix-ups when you get home. Another part of the philanderer's creed: "I will never compromise my lover's home situation by thoughtless or selfish actions." Also: Never tell anyone what you are doing, not even your best friend. "We don't encourage extramarital affairs. We understand them," Mitchell says. "People who come to this site are already sitting on the fence. I help them make an informed decision, to see if the benefits outweigh the risks." Mitchell claims he's getting 35,000 hits a day. About 70 percent of his online personals come from men, who pay about $10 a month (cheating women can post ads for free). He says women are more active than men on his message boards. Mitchell admits receiving his share of angry letters from husbands and wives who've been done wrong, but says that's less than one-tenth of the e-mail he receives. "I can't say those people are wrong. Everyone has their own moral code," he says. "It's always best to be honest, and honestly, this works for me." A philanderer isn't a bad person as much as a person who finds his marriage is missing something, and an affair might be that certain spark, he says. "You would never know if you met me," he says. "I could be your next-door neighbor." But what if Mitchell found that his wife was cheating on him — or worse yet, if she were one of the many happy customers on philanderers.com? "I guess I couldn't say much," he says. "That would be like the pot calling the kettle black."

3. Liar-for-Hire: The Perfect Alibi Agency Need someone to call home to say you need to work late? How about a service to send all your mistresses bouquets on Valentine's Day? A German company called "Perfect Alibi" claims it provides about 350 clients each month handy excuses, such as bogus invitations to weekend business seminars. Such liar-for-hire services range in price between $13 and $104, depending on the nature of the alibi, and a $35 annual membership fee.

4. Is Chatting Cheating? The advent of Internet dating over the last few years may have changed courtship more than anything since the advent of the pill. Some married folks miss that thrilling yet harrowing experience of flirting with a stranger via e-mail. This could be why so many straying spouses slip off their wedding ring and into an online persona. All online dating services say they've had trouble with married men posing as single dreamboats. Some sites allow members to post "discreet" listings, which allow them to not announce their marital status. Others, like Match.com, will boot you off if you're reported to be less than legally separated. Posting an online personal advertisement is a clear ethical no-no when you're hitched. But can you flirt in a chat room if you are espoused but filled with ennui? Interestingly, men say chatting is cheating more than women, according to a member survey by imatchup.com. Only 35 percent of ladies think online flirting is a breach of the wedding vows, compared to 48 percent of men. The problems presented by wed surfers posing as singles has opened the door to companies like Marriedsecrets.com, yet another married-but-still-dating Web site. "Thirty percent of those who use online dating services are married," the Web site claims. "Why not join a site specifically designed for you? With marriedsecrets.com, there's no excuses, no explanations."

5. Jealous Spouse Panty Raids If you think your spouse is a louse, you don't have to wash your dirty laundry in public. You can investigate yourself. The CheckMate Infidelity Test Kit, available at www.getcheckmate.com for $49.95.

The company says Checkmate is effective on both men and women, even if the man is using a condom or the woman showers after a tryst. The Web site is also marketing Checkmate as a way for parents to find out if their child has become sexually active.

For best results when checking up on your spouse, the company suggests you abstain from sex with your partner for a few days to make sure the suspicious stain came from a third party (and perhaps at a third party). To be doubly sure, the company provides a service of testing the husband's sperm and comparing it with the questionable underwear. What should you do while you abstain from sex with your alleged ratfink of a partner? Why not cruise the Internet, where you're sure to find kindred spirits looking for companionship?