Who is the biggest opponent to same-sex marriage?

Answer 1

Religion.

Alternate viewpoint:USA:

Since the querent asked "Who" ... a good answer with respect to the United States would be John Boehner, Speaker of the House of Representatives, who has seemingly appointed himself the sole Guardian of the Sanctity of Heterosexual Marriage. After the Attorney General announced that the Obama Administration believes that the Defense of Marriage Act ("DOMA") is unconstitutional and will no longer defend Section 3 in Federal Court, John Boehner responded that the House of Representatives will appoint attorneys to defend DOMA. However, the attorneys he appointed resigned after a week and there is still the issue of where Boehner will find the millions of dollars necessary to pay the attorneys. A letter Boehner sent to his arch-rival Nancy Pelosi (minority leader of the House) suggests that Boehner intends to force the Dept of Justice to pay the House's attorneys. Such a move would require the approval of the Senate which is controlled by the Democrats who are not likely to favor the plan. Boehner is the biggest national figure actively opposing same-sex marriage.

New York:

Each state probably has its own prominent opponent. In New York, that honor goes to Ruben Diaz, Sr., a self-styled minister and State Senator from the Bronx who wears a cowboy hat and quotes Bible scripture on the floor of the State Senate. Although plenty of Senators voted against same-sex marriage in 1999, they did so silently and Diaz was the only one to make a speech explaining his position. He has since been quoted as saying that a same-sex marriage law will be enacted in New York State "over his dead body."