Livre d'Or

That's true; I mean, with a big enough population, truly stupendous coincidences are going to happen some of the time. But if you resolve your plot through a really unlikely coincidence, readers are going to find it unsatisfying, even if that rare conjunction of circumstances did happen in reality.

There's a difference with portrayals of minority groups, though. It's a given that any group or subculture is going to have some bad people in it. But if you make the villain of your book a member of a minority, you're somewhat open to accusations that you think all people from that minority background are evil. There are definitely ways round this, and there are also some readers who are just going to be upset at any negative portrayal of someone from their culture no matter how sensitively written.

To take a hopefully non-controversial example: Dickens caught a lot of flak for the character of Fagin in Oliver Twist, because he seemed to imply that Jews are sneaky criminals. I was never bothered by Fagin, because when I read OT I assumed he was just Fagin, I didn't get the impression that his criminality or his other obnoxious mannerisms was particularly connected to his being Jewish. But then Dickens went and wrote Our mutual friend which has Riah as a minor character, a Jewish moneylender who is totally noble and admirable. And to me, Riah seemed a whole lot more like a stereotype than Fagin, partly because Dickens gives him a bunch of soliloquies about how it's mean to discriminate against Jews, so it's a lot more like he's supposed to be representative.

But in these examples, the problem isn't realism or lack of it. The people who thought Fagin was offensive weren't claiming that no Jews ever commit crimes. And I think the same with Fifty shades of Grey; the problem isn't whether Christian is realistic, obviously some people behave controllingly and abusively in their relationships. The problem is whether it implies a) that all people who like kinky sex are abusive in similar ways to Christian or b) that being abusive and controlling is romantic and desirable. From what I've read it seems likely that both a and b are true, but in any case it's not a realism issue.