The state gives preferential treatment to married couples when it comes to taxes...
...Not as much as you might think.
...only because there is the general assumption that both spouses are the biological parents of that child, and/or that any step parents have adopted said child.
...property inheritance...
Now here you have a point; however, any two human beings can jointly own property, thus erasing that question almost entirely (yes, this includes bank accounts).
While I personally believe it is unconstitutional since its clearly joining church and state...
I agree with this bit entirely.
Then again, marriage has always been a jointly-faceted aspect of both religion and secular rule - it was founded in early human societies to establish inheritance, lineage (hence the whole "taking his name" thing), property ownership, etc. It was considered a religious thing because religions generally outlasted kings, so it had the relative staying power to act as an authority on the subject. This is sort of why most progressive folks in the 1970's and 1980's had condemned marriage as a "curse of the breeders." Now it's just a means of sticking it to the Man, and at the same time gleaning the benefits and validation that marriage does bring to the table.
Personally? I think that sure, let the marriage laws change to include any two or more parties of any sex or combination thereof (yes, including polygamy, polyandry, whatever), but the divorce laws should change too: Jettison "no-fault" divorce. Barring proven abuse or actual crime, such things as alimony and the allocation thereof based on income should remain in place... for life. Nullify any and all attempts at a pre-nup contract. Barring (again) proven abuse or actual crime, ban both parties from re-marrying in the eyes of the state (or at least have the state not recognize any subsequent marriages). Sleep together all you want, shack-up all you want, do it in any combination thereof - but if you really want to get married, go in knowing that it is for life, and not just only as long as you and your spouse "love" each other (yes, those scare quotes go for hetero couples too.)
I bet modifying divorce laws in such a way would stop marriage from being treated as a political football, turn off a ton of people clamoring for it to be made into such, and instead make into what it was originally intended to be: a life-long bonding of two human beings for the purpose of building a family. Additionally, it would at least start to make people stop and fucking think before popping the question, and perhaps even make them become absolutely certain before they do get married. I further bet that most folks would condemn the idea right away - with the progressives at the head of the charge.