> are most an issue when a boy's regularly-accessible social unit has been whittled down to the nuclear family. That seems to be a crisis in-and-of-itself.
Quite the contrary, it seems that those who would modify and engineer culture have been pushing on everyone the idea that men are fungible. That step-daddy, or mommy's new boyfriend, or all the men in the hippy commune are adequate substitutes... and it seems like that's just never the case (at least not often enough to count on).
It may just be down to math. All those other men who might substitute just have their own lives. Your grandpa likely has other children and other grandchildren. Their focus is diluted. Uncles might be trying to raise their own families. Step-dad just thinks your inconvenient and a competitor for the attention and resources of his own children, either from a previous marriage or after he knocks up mommy.
And if that's not bad enough, think about the circumstances we tend to see in these situations. Mommy (and it's usually her, isn't it?) just doesn't get along with the rest of her extended family. Not only is she demanding extra attention for her own offspring, she's showing them all how bad she is at judging character, at planning her life, etc. It's a high risk investment in a person who doesn't look like a good investment at all, for someone they already have negative feelings for.