How to keep guests from using a bed they didn't prearrange?

[Sue174]

I need help with a situation. We have a cottage we just started renting. Maximum number of guests is TWO. We market it as a cozy, couples space. There is a Queen bed and we always assume a couple who books will both stay in the Queen. However, we also have a small open loft space with a double mattress in it. We keep a mattress protector on it and waterproof pillow cases. Our teenager sleeps there with a sleeping bag when we stay there ourselves. It's only roomy enough to sit up, (not stand) and is open to the main room below.  It is a very SMALL space (maybe 8 feet by 6?)

In all our rules and correspondence, we tell our guests that this mattress can be used IF THEY BRING THEIR OWN LINENS and let us know they will be using it. We know it won't always be a couple who rents but sometimes can be two friends or a parent and teen. However, 95% of our rentals are couples so we don't think this is too big an ask. (Of course, we do provide all linens for the Q bed.)

Since we have to take all laundry back home to wash it, I am not willing to keep linens on this mattress. Our cleaning fee is minimal and I don't want to raise it. I also don't want to do more laundry than I have to. LOL.

So here is the issue: The last few times guests have checked out (each time they were couples), I can tell they have been on this bed. Maybe it's just to sit on it and take in the view from up there. Maybe they just sat on it for 5 minutes because they COULD. Who knows? What bothers me is that I want to provide a clean mattress cover and pillows for the next guest who actually indicates they are going to use this bed. 

Do I just buy a large white sheet and cover everything up in case they do go exploring up there? Do I make a sign that asks guests to help us guarantee a clean space for the next guest by not sitting or laying on the bed unless they brought their own linens? (since clearly the last few guests didn't read the house rules around this topic).

I know this is a weird one but it bugs me so I had to come here and get your opinions. Hit me.

[Mike-And-Jane0]

@Sue174 You just have to accept that you are offering the whole space. If you don't want to charge a small amount more to wash the mattress protector then either remove the mattress or suck up the cost/time of washing the protector each time.