I know someone in this circumstance, and it comes down to exactly one issue: abortion. The spouse is Roman Catholic and cannot support abortion, so despite disagreeing with most of the republican platform, they feel obligated to vote with the party that opposes it. I had the same thing crop up in 2008 with a roommate who was Greek Orthodox and in every way one of the most progressive people I knew, but they voted McCain purely on this one issue out of religious guilt.
I vote for both bears and twinks.