A new episode of “Sesame Street” introduced a married gay couple, a segment a co-director described as a “milestone” for the iconic children’s series. In the episode, which debuted during Pride Month and just ahead of Father’s Day, the “Sesame Street” characters are preparing for a celebration. In front of a banner that says “Family Day,” Elmo, Abby Cadabby and others are ticking off a list of who will be at the party when human character Nina says her brother, Dave, is driving over with his fam