Adopted children have that protection. But who would aprove adoption to persons who are not married to each other.
Remember state wants stable households.
So it wants people to marry have children and set some restrictions on divorces.
Children are always protected equally married household or not. But the partners rights to certain thing is less than what you would have in a marriage.
Again we must distinguish between a parent, guardian and the biological father.
If a woman who is not married die then the state place the child in a foster home and the dad has basically no say in it
Yet again they promote marriage. But they want people to have children and a stable home. That is why government also provide child support to married couples and those who are not married.
So what is marriage as we understand it? It is the union of two persons as spouses, to the exclusion of all others. I think it is different things to different people. Some very religious experience, others for friendship or convenience, or to be parents… but you can distill it down to the common denominator, which is the joining of the two people as a unit legally and excluding others. Also idea that marriage evolves… it has changed from woman being property, easier divorce changed it, etc.
In the end its a government patented thing to look at it in a simple way. In the end its just a word and they can state between man and woman.
So why not just create a word for a marriage of people of the same sex. Put up protections for it and problem solved. Call it a parriage. Patent it and issue licenses for it.
In Australia I believe some states the recognize de facto relationship. A defacto relationship is called a common law marriage in other countries . Australia do not mention anywhere about common law marriages. The real problem is that marriage is a universal thing. Like the metric and imperial systems as well as the scoring system in rugby union is accepted world wide by everyone. In all countries a marriage between a woman and a man is accepted. But same sex marriages are not a universal accepted standard.
Again how much would gay people accept. Are they willing to go halfway? Then its common law marriages aka defacto relationship. Juyst rename it to common law marriage and of you go. But again its not accepted world wide so I do not advise anyone to go on honeymoon in countries like Iran, Iraq or Nigeria thinking everyone is going to accept you.