The “sin” of homosexuality is important to conservative Christians because it’s a sin they can complain about without being concerned that they might commit it themselves. Everyone and anyone might be guilty sins like gluttony or pride (which receive little attention), but homosexuality can always be framed as a sin which others commit. Even if one represses same-sex desire, one isn’t “really” homosexual unless they engage in same-sex activity — and, even then, they can repent and feel clean.
Homosexual activists, however, can forever be chastised and condemned. What this means is that the Christian Right needs homosexuality because they need some group to attack as part of their effort to restore the social, cultural, and political dominance of conservative Christianity in America. It’s no longer socially acceptable to attacks Jews, the previous scapegoat for Christian conservatives who hated everything modern. Atheist and humanists remain easy targets, but they lack the same emotional impact as gays (though all three can be and are attacked in just about all the same ways that Jews used to be).
People are scared about the possibility of gay marriage, and that includes people who are otherwise generally sympathetic to gays’ civil rights. The expansion of basic liberties to any traditionally despised group is often accompanied by fear — it could be found in response to efforts to expand the right to vote to women, for example. It’s a fear that people are going to have to overcome, however, because gay Americans shouldn’t be treated as second-class citizens simply because people have faith-based qualms about homosexual relationships. Religion is not and can never be an acceptable justification for the state to treat any group of people as inferior in any way.