Something which might interest you is how the IRS lost a challenge by a same sex couple (registered domestic partners) in California challenged its mortgage deduction rules. The condensed version is they managed to double their mortgage and hoe debt deduction limit from $1.1 million per couple to $1.1 million ($2.2 million total) each because they were not married.

For the details: http://www.bloomberg.com/news/articles/2016-08-19/another-good-reason-not-to-get-married-courtesy-of-the-irs