WASHINGTON — President Joe Biden invited thousands to celebrate at the White House on Tuesday as he signed into law gay marriage legislation before a bipartisan crowd that reflects growing acceptance of same-sex unions.Lawmakers from both parties attended, along with lady Jill Biden and Vice President Kamala Harris and her husband, Doug Emhoff.The triumphant mood played out against the backdrop of a right-wing backlash over gender issues, which has alarmed gay and transgender people and their advocates.Among the …