AGES: Kohan (''the straight one,'') is 31. Mutchnick (''the gay one'') is 29. OCCUPATION: Writing team; coproducing the NBC sitcom The Single Guy and developing another comedy for the network. HOW IT WORKS: Kohan: ''We work on every word together I don't think of it as a gay/straight thing at all.'' Mutchnick: ''More often than not, we walk in to [meet] a gay executive and two things happen. One, that executive identifies with me we're in the same club. And two, they like David more, because he's got a gay partner. We haven't experienced a bad moment yet.'' Kohan: ''Or a straight executive.''