Reddit mentions of Turfer Waterproof Tailgate/Picnic Blanket With Handles

There are few options. You just have to be prepared.

You can use waterproof underpads. They have disposable ones. Or reusable ones. [There are many kinds]
(http://www.walgreens.com/store/c/medline-washable-underpad-30-in.-x-34-in./ID=prod6013826-product). You can keep a few on hand in strategic locations, and they are easy to move around and use.

You can get a waterproof mattress pad (with a sheet is nicer but without works too) that you place over your sheets and pull off when done and you have a nice, dry bed underneath.

Wool is absorbent so you could use any wool blanket as well. Or any waterproof blanket.

Or a combination of all or some of the above works too. Especially if you don't always have sex in bed.