The truth is that marriage is a legal creation largely to deal with issues like children - and who has what responsibilities in rearing them - and property with divorce a way of ensuring that those responsibilities are met and jointly acquired property is equitably divided when a relationship ends. There's a lot of other stuff that has adhered to the concept of marriage over time or has remained attached to it from history - and some of it remains in legislation like the Marriage Act 1961 (Australia) and the subsequent Marriage Amendment Act 2004 (Australia) which defined marriage as 'a union of a man and a woman' and specifically excluded same sex marriage.
To me this is simply codifying certain religious beliefs that many may not agree with. Personally, I have no problem with people making a religious commitment - Pisces and I married in a church many years ago and that was a deliberate choice we made - but I can see no reason why that is required of everyone else. Our children married in civil ceremonies and that's fine, too.
Actually the more I think about it the more convinced I am that all the legal aspects of marriage should be covered by having civil partnerships - a secular ceremony of commitment conducted by an secular official - as the legally binding contract. By all means have the union confirmed by a religious ceremony after the civil ceremony if you wish to but make the law so that this has no legal status.
Once you institute civil partnerships you can sweep away the idea that marriage is only a union of a man and a woman - and, in my opinion, we should have done that long ago. A more humane and honest definition would mean those who don't fit into the box of heterosexual pairs could make the same commitment to lifelong partnership and receive the legal protections presently denied them.