"Although states passing legislation experienced a greater decrease in bias following legislation, states that never passed legislation demonstrated increased antigay bias following federal legalization."
It is sort of like how people never like it when their candidate in an election loses, but if the courts intervene and disqualify their candidate, their resentment of the winner is greater. Regardless of whether the courts were right or not, the fact that a non-electorate authority made the decision, increases the dissatisfaction with the outcome.
No one likes to be forced to do anything, and any perception of unfairness could easily elicit a stronger response of resentment or resistance.
I personally think when it comes to protecting rights it may be warranted, but I definitely see how the situation you explained could also be true. It's a tough thing to balance.
That said, I wouldn't be surprised if it's a mix of both. I'm sure people's ethical judgments are influenced by the law, too. It's the main reason I can think of why, for example, alcohol tends to be so very much more socially acceptable than marijuana in places where the one is legal and the other is not.
Also, it is probably "only" accelerating a change that was going to happen anyway. For example, there were times and places when divorce was legal, but did not become "accepted" socially for a generation or several. So, I don't think the causation primarily goes from legalization-->social attitudes.
But, I agree it's probably a mix.