Of America’s 7,382 state legislators, only 85 are openly gay or lesbian. They are, however, playing an outsized and often impassioned role when the agenda turns to recognizing same-sex couples with civil unions or full marriage rights. In Hawaii and Illinois, gay state representatives were lead sponsors of civil union bills signed into law earlier this year. In Maryland and Rhode Island, gay lawmakers are co-sponsoring pending bills that would legalize same-sex marriage. In New York, gay state Sen. Tom Duane is preparing to be lead sponsor of a marriage bill in his chamber later this session. “For my colleagues,...