BOSTON — The Iowa Supreme Court’s approval of same-sex marriage on Friday gave advocates an important first victory in the nation’s heartland, thwarting the notion that only the Northeast will accept it. But for now, New England remains the nucleus of the same-sex marriage movement, with a campaign under way to extend marriage rights to gay men and lesbians in all six of the region’s states by 2012. Massachusetts has allowed same-sex marriage since 2004, and Connecticut began allowing it last fall. The Vermont Legislature just voted to let same-sex couples marry, and supporters hope to gather enough votes to override a veto promised by Gov. Jim Douglas, a Republican. New Hampshire is not far behind; its House of Representatives approved a same-sex marriage bill last month. The legislatures in Maine and Rhode Island are considering their own versions, though they are not as far along in the process. Across New England, advocacy groups have been raising money, training volunteers and lobbying voters and lawmakers as part of a campaign they call ‘Six by Twelve, led by the legal advocacy group that persuaded the Supreme Courts in Massachusetts and Connecticut to allow same-sex marriage in 2003 […]