Defining relationships and marriage should not be the role of governments. Marriage is a personal choice and should not be defined by anyone other than the consenting adults choosing to enter the arrangement.
Remove all tax incentives and benefits for people who choose to enter into a relationship agreement, and you will end discrimination against LGBT citizens as well as those people who choose to remain single.
Get government out of love altogether by removing all incentives and definitions. That will make the tax and legal systems (more) fair to all.