There ought to be religious marriage and secular marriage, both legally recognized. There are churches galore that would allow you to marry two or three people just to show how progressive and accepting they are, so there should be no problem finding a hireling to perform the ritual, for a price, of course.
If you accept “marriage” being any grouping of persons without regard to their age, number, or consanguinity know that the obvious consequences are polygamy, incest, child marriage, and pedophilia - all of which are socially destructive.