The government collects tax for their own purposes and objectives, whereas the khums is a tax legislated in the Sharia taken on the yearly profits that remains after deducting the yearly expenses. The khums is legislated for specific goals which the Holy Legislator intended. Those goals are totally different from the uses of the government imposed taxes. It is natural then for the duty-bound individual to be obligated by the Sharia to pay khums regardless of whether he is paying other taxes or not.
It is permissible to enter into temporary marriage for the sake of pleasure and lust via legislated means and within religious guidelines. Among the benefits of temporary marriage is that it controls sexual relation between the two sexes within specific rules and guidelines; like specifying the duration of the marriage and the dower, serving the waiting period between one marriage and another, and the child born from such a matrimony being legitimate for the couple. There is no doubt that if such marriage was common in the society, committing adultery would be minimal.