As far as PC goes, the Jews have "holocaust" essentially trademarked, and all efforts to equate other suffering to the holocaust are rejected. Likewise, black activists in America essentially have the term "civil rights" trademarked and reject any effort to equate other movements with theirs. Jesse Jackson likes his monopoly on civil rights guilt, and doesn't want anyone else using it.
I'm not necessarily advocating homosexual marriage (actually, I really don't care since it doesn't impact my marriage), but the civil rights trademark, rather than morals, could be the motivation behind these preachers rejecting homosexual marriage.
If ever a group had the right to "trade mark" a term it would be American blacks and the term "civil rights."