As of February 2015, 37 of 50 states HAVE marriage equality (72% of all states) that allow an individual to marry the person of their choice, irrespective of gender. However, as yet there has been no progress toward allowing marriages of more than one person simultaneously. So there is still a ways to go.
Why would we want to encourage such a thing? Because we support every person's right to live a life of freedom of choice and non-coercion. Apart from preventing people from killing each other or physically harming others in some way, the state does not have the right to keep people from living the life they choose.
This includes imposing institutional moral values on the individual. Specifically, the state should not in any way be imposing a specific set of values derived from any particular religion on the inhabitants of that state. The first amendment guarantees that "Congress shall make no law respecting an establishment of religion." Therefore, state officials cannot impose their own religious values on inhabitants of the state through state enacted statutes.
While we support everyone's right to marry if they so desire, we believe there are inherent disadvantages to marriage which make an undesirable institution. See the History, Purpose and Need for Change in Marriage.
The advantages to marriage seem to be mostly financial. But even those advantages, in our opinion, can be gotten around with proper financial and legal planning. So apart from the obvious feel-good reasons for wanting to marry, no one should feel obligated to do so. Marriage is no longer needed in order for one to raise children or have a meaningful life and/or spiritual existence.