Well, when your partner dies and the State doesn't recognize your marriage, you don't get survivor benefits. It's conceivable to be with a partner for most of your life, and then lose the house you lived in to your partner's bigot family. That's something to feel strongly about.
That said, death threats are always disproportional, and I think we can all agree that however you feel about Eich and his contribution, they were uncalled for.
Well, it's the world we live in, not as it should be. The government definition of marriage and the rights it confers (not just survivor benefits, but substantial tax/program eligibility economic advantages, among others) are real and should not be withheld on the basis of sexual orientation. And the priority should be addessing this clear violation of the 14th amendment before addressing the relationship between the religious ritual of marriage and the nondenominational legal relationship of marriage that regrettably shares the same name.
That said, death threats are always disproportional, and I think we can all agree that however you feel about Eich and his contribution, they were uncalled for.