Well they're not going to ask "Is the seat between Mr and Mrs Jones all right?", there's no need to expose the name to you when you ask to be placed between people with the same last name.
Before the middle seat person gets on the plane, they only know that there are two people there who are married, not that you specifically are one of them. The existence of married people is not a secret.
After you're on the plane, it would probably become apparent that you and your spouse are married, or at least related in some way to each other. If you have some game with your spouse where you pretend to not know each other in public or something, I guess maybe it could ruin that, but otherwise I'm not sure what the harm is.
