@Carol249 - You have a couple of options:
1. you could charge for the privilege of sleeping on the couch. We do - an additional $50 per night for the linen, blankets and extra clean up. Most people, when told of the cost, choose to sleep together - even adult children and their elderly parents.
2. You can be firm and tell them no during the reservation process - can't tell you how many times someone asked if they could sleep on the floor in the guest room - no, no, no. Of course, we live here and you will have a much harder time policing where people sleep with the private home as @David126 suggests. That doesn't mean you have to provide bedding. Your listing clearly states 2 beds and does not list the couch as an option. Once someone has made a reservation, I would ask who is coming and remind them, if the pairings in bed seem dubious, that there are only 2 beds in the home.
3. Add the option of an air mattress at an additional cost. If someone wants the extra sleeping space, you could insist on this as the only way and then you can set the bed up as you like. Again, you can charge for this (post it right in the listing) . I would probably say something like: Not sharing a bed? We insist on providing you with an Airmattress for an additional $xx/night for the extra sheets, blankets, pillows, clean up and set up." That way anyone reading your listing will be told right up front what you consider acceptable in your home.
Good luck!