Bay Street Theater officially kicked off its summer season on May 31 in Sag Harbor with Bob & Jean: A Love Story, playwright Robert Schenkkan’s unabashedly romantic homage to his parents. As its title makes clear, this is every inch a love story. But don’t be misled. Yes, it’s light and airy, but it’s in no way insubstantial; it’s optimistic but not naive; elegiac but not maudlin, old-school but not old-fashioned.