One-Third

http://www.theguardian.com/world/2013/dec/19/new-mexicos-highest-court-rules-gay-marriage-legal

With New Mexico joining the 21st century, by my account a third of Americans currently live in states that have legalized same sex marriage.  That does not include states like Colorado and Wisconsin that have civil unions and partnerships that figure to become the next states with legalized same sex marriage.

After that, the low hanging fruit is kind of gone.  You would need a major shift in the structure of power in the legislatures of Michigan and Pennsylvania for them to embrace equality.

Otherwise, it will come down - as it did in New Mexico - to the Courts, and eventually the Supreme Court will have to strike down DOMA.

Some day soon...