Even if those issues were to be resolved somehow, we still have to deal with other nations that do have state recognition of marriages. For example, say you wanted to adopt from another country, you would not be able to do so in many countries because they specifically require foreign adoptive parents to be married, and expect state recognition of that marriage. The same would be true with tax laws in other countries (say you worked abroad), property ownership in other nations, and so on.
The fact is, its completely unworkable for the state to simply not recognize marriages at all. Its also rather petty really. Basically, its like saying well if gays and lesbians can get married, then the state should just quit recognizing any marriage. Thats the kind of thing a 5 year old would do when they did not get their way.