Today's typical example of marriage is two people who are legally united, probably live together, have kids and wear wedding rings. It is the legal process however that technically equates to marriage in our society, but as you and MGOwen pointed out, marriage goes back before government.
Therefore I would suggest that marriage is the decision made by two people to faithfully live their lives together and support one another through thick and thin (physically, emotionally, sexually, etc.), and for better or worse as they both strive to overcome and conquer the many obstacles and complexities of life. The specific actions tied to this decision (ex. making a marriage legal) are determined by the culture and time period in which the decision is made. However regardless of these factors, those who are married are making the choice to watch each other's six during the battle of life, so to speak. In summary, marriage includes the decision, and the continuous fulfilment of the vowed and implied terms of that decision.
Some of the options you gave aren't marriage per se but an argument may be made that they should only exist within the context of a marriage (ex. sex or having kids), hence the possible conclusion that these things equal marriage by implication.