In the five months since they took control of the House, Republicans have banned government-funded abortions in the District, revived a private-school voucher plan and tried — so far unsuccessfully — to end the city’s needle-exchange programs. Yet despite past efforts, Republicans have not mounted an assault this year on the District’s same-sex marriage law: No bill has been introduced to overturn it, nor has any lawmaker publicly sought support from colleagues for such a measure. Del.