Sam says that he's never had a woman as just a friend. That's not true, as Carla is a friend, and they've never been together romantically.
Sam tends to overlook Carla quite a bit. Besides, she started out as a co-worker, not a woman he met socially.
Diane: You think I don't know what noodles mean in sexual lingo.
Heather: I thought we were having Veal Oscar?
Diane: There are remnants of it in the sauce.
Diane: Do you have reservations?
Sam: Plenty of them, but I can anyway.
Heather: I sure like Sam.
Diane: I have a special recipe to try, my own Veal Oscar.
Carla: That's the award you get if you can act like you like it.
Cliff: Yup. Look at that Norm. Just like the rest of them. All hung up on looks, personality, and style.
Diane (to Heather): That's Carla, she likes witty repartee. She's just not capable of it.
Diane: How can you be eating now?
Sam: I've eaten through worse than this.
Sam: I'll tell you something else I haven't had much experience saying no to women. The closest I've come is "Not now, we're landing."
Diane enters Cheers with a friend.
Diane: This is Heather Landon, my oldest friend.
Carla: Meet her this morning?