All taxes are social engineering. If you want to disqualify all tax benefits that equate social engineering you’d have to disqualify all tax breaks.

As for separation of church and state (let’s presume for arguments sake it’s a real thing) marriage in the legal since is not a religious act it’s a contractual commitment between two parties (the ultimate reason banning same sex marriage is a doomed policy) that’s completely separate from the religious side of the act. Non religious people can enter into a marriage So it’s not an exclusively religious act.

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