Gay couples have helped strengthen the institution. Money quote:
Seventeen years after recognizing same-sex relationships in Scandinavia there are higher marriage rates for heterosexuals, lower divorce rates, lower rates for out-of-wedlock births, lower STD rates, more stable and durable gay relationships, more monogamy among gay couples, and so far no slippery slope to polygamy, incestuous marriages, or "man-on-dog" unions.
Who would have thought that encouraging marriage for everyone would do all that? Ahem.