Senator John Kerry and Gov. Deval L. Patrick left early. But Representative Nancy Pelosi stayed late — and swayed on the dance floor to “It’s Raining Men.” Elizabeth Kucinich snapped photographs, as her husband, Dennis, chatted up Representative Steny H. Hoyer by the hors d’oeuvres. And Terrence McNally, the playwright, joined in the hora and a group singalong of “Low Rider” by War, with President Obama-themed lyrics written for the occasion. There was no shortage of boldface public servants here on Saturday, when Representative Barney Frank was married in what one guest described as “the wedding of the century for liberal gay Democratic politics.” And the reception, with music, vegan dishes and general sentimental gushing, offered its own display of the liberal Democrat establishment celebrating the first-ever same-sex wedding of a congressman.