Do kids justify selfishness? Why?

I have a friend who just submitted her thesis for her PhD in the history of the early christian church. I helped proof it, and there was one really interesting thing that stuck with me: a lot of early christian writers thought christians shouldn't have families, because families foster selfishness - e.g. "I've got to do what's best for my kids, even if it hurts other kids" or even just a general "us vs them" mentality, where christians were supposed to care for everyone, not just those they happened to be related to. 

And as weird and uncomfortable as it feels to think about it, the more time I wrestled with it, the more I kind of think they had a point. People ARE selfish when it comes to "them and theirs," and are completely unapologetic of it- it's a point of pride, even. You see it all the time (especially here): people defending selfish choices, hypothetical or otherwise, on the basis that their families - especially their children- are the most important things. I actually see that attitude praised a lot, as if you're a bad mother if you aren't totally ruthless in your dedication to your children and their safety. 

So: what do you think? Do kids justify being selfish, putting your needs over the needs of other people, and other people's children? Is it noble? Is there a line between what is acceptable and unacceptable selfishness? How do you define it?