We should continue to limit licenses to have sexual intercourse (aka marriage) to:
of Opposite Sex
Two in number
Who are not closely related.
The only change that needs to be made in our marriage laws is to make a marriage much more difficult for one party to dissolve.
Our society, in order to be just, needs to acknowledge the interests of children in having a Mom and a Dad who are married to each other for life.
To have a just and caring society, anything less must be acknowledged as a loss.