Why are men so afraid?

Maybe it's because there are still a few of us grown boys out there who actually believe in the naive romanticized notions of 'love' and feel, when one professes to 'love' another, --as she did many times in the film-- that that type of infidelity is simply wrong.

She has her cake, and she's eating it too.
Then she see's another cake and wants to be greedy.
Much like a pig.
The same thing could be said if the protagonist were a male.

But sadly these are just the childish notions of a man-boy who has been bombarded with manufactured ideals of 'love' implemented by actual grown-ups that think up the age-old stories, songs, poems, and other media most of us are exposed to as we live life.

Immaturity is part of all our complex personalities at some level. We're all imperfect (this sounds like religious proselytising but it's not!) and that's why we can sympathise with her. Who hasn't acted foolishly in love, and made mistakes over and over again, whatever age we are?

"Her "cheating" is not superficial or shallow but is part of her complex, confused personality"

Why do you put cheating in quotation marks? How is cheating on a spouse anything BUT superficial and shallow in general (not to mention the supreme douche bag she cheats with in this movie)? Using your line of thought, one might argue that women that relate to the protagonist of this film are "afraid" of admitting to being 'not so great' human beings. The Michelle Williams character isn't "complex" at all. She is an infantile, whiny bore, who happens to be sexually attractive.

The way I see it Margot isn't someone you're supposed to fully sympathize with.
That is very clear especially in the last scene when she walks away from Seth Rogens character with tears in her eyes. Probably realizing how emotionally conflicted she is and regretting her "mistake". You can see how torn up she is when she is about to cheat on Rogens character.
This film isn't about moralities. Relationships aren't made of steel, love is a fragile thing. It is clear from the start that their relationship isn't working perfectly. If you dislike the film because you are unable to put aside your own fear of being cheated on then whatever. Still one of my favorite movies ever.

Ged59, you've put up the question about (all) men being so afraid, but upon every reply in this thread your counter-argument is the SAME ONE: people should not be made responsible for their decisions.
Perhaps, the "devil made them do it"?
I'm certain you wanted to bait people by declaring that people "who miss all the depth in this movie" are "just idiots", but whenever there was a reply, all we got from you was a mantra that's not illuminated with examples, but simply repetitive.
If you want us simply to commend you on your unique respect for the "depth in this movie" just say so. Then nobody wastes their time here under the pretense of discussion and argument.
Coaching tip: you want to own this board, work on your rebounds.

Why couldn't her husband just try a little harder? There were so many signs. If you see your relationship/partner slipping away, why not do all within your power to salvage it/them? He didn't even fight for her.

Why can't you accept that you are simply a whore apologist?

Why do you put "cheating" in quotation marks?

I agree that the double-standard thing is dubious from the get-go (I'm not all that convinced by the proposition that male cheaters are almost universally seen as negative in films, mostly because I could start naming one instance after another where that's not true).

If people are reading this as some kind of "she cheated, and it was just fine" kind of film, boy, are they ever missing the point. I agree completely that she suffers a lot. So does Lou. So does the "other man." So do the friends. So does her sort-of-ex niece. And on and on. The film is an examination of how easy it is to slide into cheating, one toe at a time, just getting a little intrigued and then finding yourself blowing up the place, and yourself with it. It's real pain. IMHO, it was excruciating to watch, and incredibly valuable, also. But if you're saying that people are missing the point by seeing this as an individualized issue of what Margot did or didn't do, or whether it was a lark for her or it wasn't, yeah, I think that's right. It's an examination of the state of adultery. It even leads to questions about marriage itself -- like, do you really mean it when you say "forsaking all others"? Forever? Until you die? Most people, I think, don't even think about that very deeply. You forego all kinds of other paths, roads not taken. In a way, it's an acknowledgement of your own limitedness and your own mortality. And then, if you don't want to create mass pain in ways you haven't even thought of, you have to stick to it. One and only one, until you pass from this world into whatever comes next, or nothing. Makes you take a big breath even to think about it. That, to me, is exactly what the film is trying to do.