Whether you have sex naked or with clothes on, you don't want your futon to get dirty.
I'll tell you what to do if you have that problem!
Your bedding will not get dirty during sex without bathing.
What does get dirty is probably fluid-related.
It could be your bodily fluids or sweat, your partner's sweat, or Cowper's fluid, which is called patience juice.
If you are very wet or squirting, you may end up with stains.
Getting wet is not a bad thing. It's just that your body reacts to it and it feels good.
So what should you do?
If you are worried about stains, you can put a bath towel on top of the sheets, or pull a terrycloth over the sheets and replace them.
It is also washable, so sweat and other stains can be washed together.
Also, you can use :
Pet's pee pad
Water Proof Sheet
You can buy $30 and above if you are based in the US, you can reuse.