Equal rights and protections for all under the law are not special interest politics, unless you are working under the guise that homosexuality is something someone chooses.
Marriage is a special interest class (A). The debate is whether or not to expand special interest class (A) to include other special interst classes (Z) (ie, make it larger A'=A+Z). Merely voting to include yourself in the privledge class does not make you some sort of prophet of truth and justice.
Now some disagree that it should be "of the opposite sex" and instead want it to be: Everyone is allowed to marry one person.
Soon the next special interest will come along and want it to be: Everyone is allowed to marry one or more persons.