"If the 'marriages' are 'spritual' instead of legal, is that different than for a man who is married and having an affair?"
Not if the women the man is having an affair with is of legal age. But if she is, say 14, then that is called statutory rape at a minimum.
"But, if underage girls in America have sex, men and women 'date' and have sex, many people marry-divorce-marry-divorce and are polygamist sequentially---it's hard to decide what is technically legal and what is 'frowned on'."
What is so hard about it? An adult male (or female for that matter) having sex with underage girls (or boys) is illegal. A man possessing more than one wife at the same time is illegal.
"I think removing a person's own child should be the last resort."
I agree with you on that one, as well as the fact that the state was overzealous in removing all the children without cause. However, I don't know all the facts on the decision making process of the agency involved. I would hope they did not enter the compound with intent to remove all children, but rather to investigate the claims alleged in the phone call.