Finally, The liberals' strategy on this issue is and has been quite simple:
1. Get a few states to legalize SSM on their own (this step is now complete)
2. Get some judges, including from places that oppose SSM, to legalize it by force (this step is now complete)
3. Scream "precedent! precedent!" at the SCOTUS until they force the entire nation to do things the same way.
The liberal strategy on this has been quite clear: Attain equal rights (of which SSM is a facet) for LBGT citizens, by any means possible.
You realize that "slippery slope" is a
logical fallacy, right? Democrats, and Americans at large, do not support incest and polygamy in the present. Maybe 50 years from now the social consensus will change for those things to be acceptable, but there is not some secret plan to make it happen.