Characters: Acceptable vs. Unacceptable Flaws

I should preface this post by saying I've made it a personal policy to avoid my books' pages at Goodreads.  Readers resent writers who are constantly peering over their shoulders, and I don't need to be driven to distraction by all those ratings and reviews.  This means I'm pretty much in the dark about how my work is being received there.

Anyway, one of my fellow Dreamspinner authors recently let me know she read and liked precious_boy . . . then noted how some other readers' reactions were illustrative of my previous blog post.  Specifically, said readers were troubled by you-know-which scene involving the protagonist and his former lover and downgraded the book accordingly.

Okay, no surprise there.  Some early reviewers were also bothered by the scene.  But this post isn't about that novella.  Rather, my fellow author's reference to reader reactions got me thinking. 

The vast majority of m/m romance fans claim they prefer flawed characters.  But what does "flawed" mean?  Clearly, there's a difference in people's minds between acceptably flawed and unacceptably flawed.  So, which weaknesses, evils, mistakes, lapses in judgment are acceptable and which aren't?  Does a reader's degree of leniency depend on the author?

For example, most readers were extraordinarily forgiving of Tor in Bareback when he went on his secret cheating spree.  Ditto Jake in the AE Mysteries when he flung Adrien around.  Those behaviors -- extended infidelity and physical abuse -- apparently fell within the parameters of "acceptable" fuck-ups, because they didn't significantly alter readers' favorable impressions of these books.  There've been other characters in m/m romance who've also been guilty of some pretty egregious transgressions -- like rape and murder.

Back to my original question.  How flawed is too flawed?  And in which ways, under what circumstances?  What exactly are the limits of character fallibility in m/m romance?  What makes readers cross that line from admiring a story to scorning a story based on a character's actions?  Is there an element of mob mentality involved? (I really don't like what happened, but everybody else seems okay with it so I guess I should overlook it too.  Or the reverse of this.)  Are some authors given more of a pass than others, no matter how badly their characters behave? 

I think this is a fascinating topic, although it leaves me totally bewildered.  Enlighten me!