"I would have thought third marriages were a pretty strong nail in the coffin of marriage myself"
What do you mean, jno?
If, as is suggested, the benefit must be paid to all couples to ensure the welfare of children of unmarried couples are not disadvantaged, the simplest way to deal with the alleged discrimination against unmarried couples would be to simply dispense with the benefit entirely. But of course this would be couples with dependent children (whether married or not) would have to make their own arrangements to cope with tragedy - such as buying life insurance.