Study: banning gay marriage harms traditional marriage

FiveThirtyEight did the math and found that, while the overall divorce rate in the U.S. has declined in the past several years, the decline has proceeded more slowly in states that have enacted legal bans to gay banns. In other words,

[t]hose states which have tended to take more liberal policies toward gay marriage have tended also to have larger declines in their divorce rates.

In other words, it's not same-sex marriage that harms traditional marriage. What actually harms traditional marriage is putting up legal roadblocks to the civil recognition of same-sex partners' happiness.