Basically, men cheat* because:

1. Their penis told them to.
2. The penis is always right.
3. Unless it's not. Unless it's totally, blindingly wrong. But that's really not possible. Just ask it. Wait a sec, it's busy with that Vegas waitress. OK, go ahead. See?
4. The man's marriage is sexless and loveless and boring, and he has needs that must -- nay MUST -- be met. Just ask the penis.
5. The wife has low/no libido, whereas the man has enough for nine teenagers and a box of rabbits.
6. No, really. The male libido, generally speaking, far outpaces the female libido and is never really satisfied for more than a day or two, tops. This is why so many men choose to be gay. Gay sex is like, off the hook! It's true! I read that somewhere. Lesbians, on the other hand, often suffer a terrible fate known as "lesbian bed death." You can Google it.
7. #6 is a totally unfair cliché that doesn't always hold true, and, by the way, all men are pigs. I mean, duh.
8. The kids steal all the love/attention/energy from wife, leaving man with nothing but XTube, golf and vodka.
9. One word: Ego. Three more: ego, ego, ego. Nothing like nailing a beautiful female acolyte to boost self-image, over and over again, as she coos your name adoringly and feigns surprise that you just bought her a Mercedes.
10. Or is it the other way around, and many women are shockingly predatory, often hunting for rich, powerful men who will buy them stuff and give them a shred of self worth by association, because the truth is, they have no identity of their own, and all they have is sex, which they wield like an ax made of lip gloss and open hip flexors and Cosa Bella thongs?
11. Man is a rock star/golfer/politician/televangelist and women -- or gay prostitutes -- are knocking on his hotel-room door day and night, and the penis is like, are you going to answer that?
12. Man's marriage is basically a sham, held together only for the sake of kids and media and multimillion-dollar Nike endorsement deals, and to pay for mansions, guitar-shaped swimming pools and giant chrome rims for the Escalade.
13. Sex is tasty and delicious and should not be denied to mere mortals like weak and meager little men.
14. Man is insecure, and sex makes him feel wanted and powerful.
15. Man is very secure, and sex makes him feel even more wanted and powerful.
16. Man is impervious. He cannot possibly get caught.
17. Man is impervious. He does not care if he gets caught.
18. Wife won't do various kinky/perverted things man has taken a liking to from scouring Interweb at 3 a.m. whilst moderately drunk and naked. Wife has zero sex skills. Wife hates sex. Wife only grudgingly has it when she has to to shut up his whining. Wife is disgusted by his dirty suggestions for the new video camera. Wife has let herself go. Wife will only have sex if it leads to more babies. Wife is Sarah Palin. Mix and match.
19. Sex is not love. Cheating is not really "cheating," per se, given how most men consider casual sex romps to be just slightly above "meatball sandwich and an ice cold beer" and just below "hitting 210 mph in Porsche Carrera Turbo while tripping on acid in a lake of fire" in the Male Desirables Index.
20. Man has become convinced the human animal is not meant to really be monogamous, that fidelity is for Hallmark cards and Mormons and has no bearing on real life. Man is simply not wired to mate for life. Hey, it was on the Discovery Channel!
21. Wife actually gave man permission that one time when she was really drunk after being handed keys to new Bentley and a house.
22. Except she wasn't drunk at all and knew exactly what she was doing.
23. And so did he.
24. Quit making excuses. Man is a pathetic, sexist jackass and always has been, basically.
25. Which makes you wonder why she married him at all.
26. Maybe she thought she could fix him. That's pretty common.
27. Or maybe he was very, very convincing when he said he'd changed, that his playa days were over, that he loved her and needed her and never wanted to have sex with anyone else, ever -- no really, I mean it this time.
28. Which was probably true.
29. When he said it.
30. And she wanted to believe it so badly. "Maybe marriage would change him? Or a couple kids? He's ready to settle down, I can feel it."
31. Even though everyone around them was like, "Oh my God, that's a disaster waiting to happen, right there. He's such a letch. Why doesn't she see it? Should someone tell her? Is she just not very bright?
32. "I mean, he just totally hit on me at their Christmas party. And she was like, 10 feet away! Of course, I slept with him. But never mind that now!"
33. Let's flip it over. Maybe wife is a ruthless, nightmare harridan, relentlessly negative and mean. He can do no right. He is lonely and starved for attention. But they have kids, a home. Divorce is ugly, expensive, public. So...
34. Oh, stop it. There is never any valid reason for cheating, even if she's an abusive monster. There are only excuses.
35. Thank you, Elle magazine. You are childish and wrong and do not live in the real world. Go back to your pink pedi and "Twilight."
36. Bite me.
37. Man is entitled. "I deserve lots of casual sex. After all, I didn't work this hard on my business/golf game/these abs not to have them licked by a wide variety of giggly TGI Friday's hostesses. Wait, did I? No. No I did not."
38. Porn made him do it.
39. Sex addiction. He's a victim.
40. And a sinner! We are all sinners. Who are you to judge? Sinner! You! Now take off your pants and get in the goddamn hot tub already.
41. Man fears mortality.
42. Man fears erectile dysfunction.
43. Man fears fear.
44. Man fears deranged fan will beat him/kill herself/post photos on Facebook if he doesn't have more sex with her. What's he supposed to do?
45. Man is getting back at his mother.
46. Father.
47. Priest.
48. Invisible friend.
49. Invisible friend's priest's mother.
50. Wife has tacitly agreed to don't-ask, don't-tell policy WRT his fooling around, and is not at all unhappy with having $20 million in her checking account while she never has to have sex with her husband. Hello, American dream!
51. Organized religions and entire conservative platform essentially say that women are lesser, lower, should be kept in their place, and that place usually involves denial and alcoholism and blind acceptance of your man's wanton indiscretions, because he's the man and that's all there is to it, so shut up and take another Zoloft and keep your crying to yourself. Yay, GOP!
52. Didn't Jesus fool around? Is that written somewhere? The lost Gospel of Hey Baby, Nice Rack? All those prostitutes and magic and hocus-pocus? I bet he did. Dude could walk on water. Chicks love that.
53. And by the way, isn't cheating sort of God's will? I mean, He's omniscient and everything, right? That means He knows it all before it happens, it's all predetermined and fated and a priori, and therefore he knows we're gonna cheat, right? So it must be OK.
54. Hey, temptation is irresistible. Who can say no to a secret illicit romp on the office conference-room table?
55. ... or on the boss's desk?
56. ... or in the principal's office?
57. ... Wal-Mart parking lot?
58. ... iHop walk-in freezer?
59. ... 1995 Chevy Caravan third row fold-down seating?
60. Men don't cheat, actually, at least not nearly as much as the culture/feminist theory thinks.
61. They actually value and cherish emotional connection just as much as women. It's true. Media blows it all out of proportion. So not fair.
62. So essentially, we're talking about the classic, time-honored breakdown in communication and gender understanding, exacerbated by horrible sex education and Dr. Phil's BS and endless lies from fashion magazines and Oprah and porn.
63. Actually, the headline of this column is sort of misleading. Men don't cheat. Some men do, and some women do, for all sort of reasons, some of which are actually sort of valid, if you will, and to insist on some ironclad universal rule of absolute unquestioning fidelity is to presume a ridiculous, impossible level of perfection in the human animal and to dismiss the million messy, complicated variants a human love relationship can take.
64. Oh, just shut the hell up, #63. No one wants to hear your tepid, permissive psychoanalysis. Cheating is wrong. Always and forever. Now let's talk more about drunk jerks and skanks!
65. Hey, I'm just trying to provide a little perspective, rein it all back in. This is getting out of hand.
66. Whatevs. This entire column is built around a totally ludicrous and unanswerable question, anyway. Sure, there are as many reasons for infidelity as there are human emotions. Life is messy. Love is messier. But mostly it's about the penis.
67. Yes, but ...
68. Just stop it.

69-100. Something to do with monkeys.

101. Love is not your bitch.

* NOTE: Many of these also apply to women who cheat. So don't kid yourself, sister.

Mark Morford's column appears every Wednesday and Friday on SFGate. Contact him here. To get on the notification list for this column, click here and remove one article of clothing. To get on Mark's personal mailing list (appearances, books, blogs, yoga and more), click here and remove three more. His website is right here.

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