I never claim my tithe in taxes. What I give is between me and God and I do not do it for a tax break. The Bible says to give without expecting anything in return.
I suspect there will be many who will remain faithful. I also believe that many, and possibly many more, will stop tithing once they can no longer claim it.
I can say the same thing, I tithe because God's Word tells me to, not to claim a tax deduction.
But even so, I maintain that government has no constitutional right to use taxation as a tool to force a foundational change in the most basic institution of human civilization, i.e., biblical marriage. I don't believe that God will tolerate this abomination without severely punishing, or even destroying, any nation that approves and legalizes it, and ours won't be an exception.