This argument makes no sense. Why would extending marriage benefits to couples regardless of gender allow groups of people to marry? There's no reason why a "nonmoral neutral contract" could not be limited to two individuals based on the fact that it is a partnership contract.

If you want to argue that governments shouldn't sanction marriage because it is the domain of religion, fine. Good luck on that one. But allowing two individuals to marry regardless of race or gender, as is mandated by our constitution, does not lead to a slippery slope.