The only thing I can think of is that "expediency worked". OmegaDad told me he thought the sponsors of the bill were idiots for having two clauses--the first defining marriage as between one man and one woman, the second banning any legal union purporting to give the same legal benefits. He thinks they would have won if they had been separate measures.
Part of me would like to think it's the wild & wooly anti-government individualism--akin to Libertarianism--that lurks in the backwaters of the state.
The other part of me is just pouting that their lousy motto--"Takes away healthcare"--won the day.
It's definitely going to be contested; the result is close--51% to 49%. But still...