In 2003, more than a decade before it legalized gay marriage, the Supreme Court declared all anti-sodomy laws unconstitutional. The United States could no longer punish a same-sex couple for engaging in homosexual acts. And that, combined with Roe v. Wade, is when America ceased to be a Christian nation.
You have a choice whether to participate in the public sin of others. If you join it, you show others that you approve of that sin, and you encourage them to do the same. Which is why Christians must refrain to the best of their ability from participating in gay weddings.
People who are conservative in a general sense find it especially disturbing how the unintended consequences of government decisions ripple out into economic decisions, which in turn ripple out into societal change. As much as the secular complain about the influence that religion has on society, look at the influence that *government* has on society, and consider how unthinkingly it spends that influence.