Little light reading to help you sleep at night?
Both are correct. In the first example the singular form is used because it's accepted that either their husband or boyfriend is the one abusing them. Not both or multiples. Of those 3 millions women, each one is abused by one of the two choices.
In the second example. All the women were killed by a husband. Not a single boyfriend was taken into account. So the plural is used to put them all is the same category.