The gay marriage wedge.

Why are Christians so obsessed with homosexuality and gay marriage? - Beliefnet Forums:

"I think homosexuality and gay marriage are simply the politically correct whipping boys for those Christians that accept the Pauline principle that sex is for procreation only and any other expression of sexuality is sin. It would be hard to generate a political movement against recreational sex in general but homosexuality and gay marriage are wedge issues that can establish the legitimacy of political regulation of sexual activities.

First the gays, then the fornicators, and pretty soon cameras in the bedrooms to insure no contraception and the missionary position only."