CBS has announced that an upcoming plotline on “Two and a Half Men” will involve two straight men, played by Ashton Kutcher and Jon Cryer, posing as a gay couple in order to adopt a baby. Somehow, that sounds familiar. After a fraught history of gay rights in America — and a fraught evolution of gay characters and themes on network TV — it seems our culture has finally come full circle. And in a clumsy, tone-deaf way, CBS is proving how mainstream gay family life has become. The idea of being gay was once casually dismissed on network TV. Now it’s presented as something to covet. Here’s a brief, incomplete history of gay-related sitcom stories.