"So it would be best to officially secularize the state-concerning legal aspects of marriage, and let various churches, sects and suicide cults conduct whatever rituals they want for who they want on whatever basis they want."
This is already how it is done. You go to city hall or county office, etc, to get a marriage license. There is no requirement to go to a church, sect, or cult, but you may not be viewed as married by those institutions. Similarly, if you only go to a church/sect/cult and don't go to city hall, you're not married in the eyes of the law (exception: states with common law marriage).