Its an uncomfortable fact that marriage as a concept is, on a population level, a force for good within society. However 'outdated' we like to think it is, the fact cannot be avoided that it promotes social cohesion, stability and (indirectly) boosts GDP
Furthermore, the outcomes for children raised in a home where the parents are married to each other are better across the board than they are for comparable households where the parents are not married to each other.
The State does have a responsibility to 'nudge' us into behaviours that are good for society, so restricting benefits like this to households where the parents are married to each other are, on a macro level, of benefit to us all.
I think we are in danger of becoming so woke on an individual level that we lose sight of the bigger picture sometimes.
While on the subject, I will say that in my decade of volunteering at an advice giving charity, I have never (and I do mean not even once) come across a woman who would not be better off married to her male partner than remaining unmarried.
This admittedly does stem from the patriarchy, but it is important that we tackle the facts as they are not as we would like them be.
I'm not suggesting that these women don't exist, but they are few, far between, and usually at the upper end of the socio-economic spectrum (hence not quite so in need of support)