Taboo-breaking Uruguay is poised to legalize gay marriage, with lawmakers planning to vote Tuesday to create a single law governing marriage for heterosexuals and homosexuals. It also would let couples, gay or straight, decide whose surname goes first when they name their children. The naming proposal breaks with a tradition that has held for centuries across Latin America, where in nearly every country, laws require people to give their children two last names, and the father’s comes first. ‘‘It’s an issue that will generate confusion in a society that has forever taken the father’s name. But these changes in society have to be accepted,’’ said Deputy Anibal Gloodtdofsky of the right-wing Colorado Party, who told The Associated Press he plans to join the ruling Broad Front coalition and vote in favor on Tuesday. Legalizing gay marriage has long been a goal of the Broad Front, which has ample majorities in both houses of Congress. After the lower house votes, the project would go to the Senate. President Jose Mujica hopes to sign it into law early next year.