Your number 3 is worded a little weird. The only reason a woman’s life and her child’s life would be in jeopardy if she found herself pregnant without a husband is if she were rejected by both her family and the father’s family. That’s a cultural issue that goes beyond the need for more bodies for defense.

What about already born siblings? Reject the mother, you reject them too. So this situation only works for a newly married first child situation or unwed mothers in the first place.

Also, before the advent of modern medicine childbirth was a risky venture. Each and every one. Full stop. Reaching the age of 5 was an accomplishment that in some cultures earned you your name. That’s right, some kids weren’t given a proper name until they reached the age of 5 because they were expected to die. Under such dynamics, it makes sense to try to preserve each and every life; there’s no way women and children were that disposable if you’re trying to increase or stabilize the population. The civilization in question simply can’t afford to think like that without some other dynamic going on.

So there’s some incongruity of thought here. That other dynamic is most likely line of succession and inheritance. Full filling tribal obligations and expansion through arranged marriages may have also been a factor.