In Pennsylvania, Quakers can be legally married with a Quaker marriage certificate, which if you are familiar with the Society of Friends, doesn't have an officiant. This is an example of a different form of union than the traditional "marriage" with an officiant. But non-Quakers can get legally married with that Quaker marriage certificates as well.
If we created a separate civil contract of union, without abolishing marriage in its current form, and have that option for everyone (homo or hetero), then that's how we can sneak it in. The idea of abolishing marriage first to be replaced will never happen, since it's so rooted in religion. Even people who aren't against gay marriage would probably be against replacing marriage with civil contracts of union.
I hope that all just made sense.