Since it's about the civil aspects, same sex marriage is the logistically easier topic.
Take taxes: tax codes cover singles and pairs, optionally with children. Same sex marriage doesn't change much here, polygamy does. The same for lots of other provisions that are affected by civil unions (no matter what they're called).
It can be done, but it's a lot more work (and as such, a lot more legal attack surface) than declaring "we don't care anymore for the sex of the pairs' members".
Take taxes: tax codes cover singles and pairs, optionally with children. Same sex marriage doesn't change much here, polygamy does. The same for lots of other provisions that are affected by civil unions (no matter what they're called).
It can be done, but it's a lot more work (and as such, a lot more legal attack surface) than declaring "we don't care anymore for the sex of the pairs' members".