The large and vocal constituency activated against the bill is good evidence that in fact it is not already accounted for under existing civil rights law.
Precisely. I'm trying to understand the resistance in good faith, but the article didn't explain their position very clearly. "It's already prohibited" - so why the fierce opposition? "Passing a law against discrimination of X will stigmatize X" - Imagine someone was making this argument for an existing federally protected class.
Moreover, every new law carries a risk of having unintended consequences. So the argument may well be as simple as, if it ain't broke, don't fix it.
(That said, I don't get the impression everyone who opposed it did so for the same reasons.)