So what? He also supported the Civil Rights Movement, started the Peace Corps, signed a limited test ban treaty, and signed landmark legislation to improve care for people with intellectual disabilities. He also signed the Equal Pay Act of 1963, abolishing sex-based wage disparity. No modern Republican would be caught dead supporting any of these things today.
I'm sure he joined the NRA back when its primary focus was on hunting safety. They're now a fundraising arm of the Republican Party. Even old man Bush tore up his membership card in protest after the NRA called federal law enforcement officers "jack-booted thugs" in the wake of Waco and Oklahoma City. Kennedy would have done the same.