I agree replacing sheets is indeed part of the operating cost. If someone has an accident or makes a small stain obviously you have to just go get a set of new sheets.
But would you bleed over someones sheets, not mention it, refuse to contribute to the cost, and then want to go back to that place?
I've seen lots of posts here about women who have had menstrual accidents in hosts bedding, sometimes quite severe, and then act as if it's normal not to pay for blood saturated ruined sheets?
In this case, these are repeat guests, I think @Susan378 has several choices: (1) don't have these people back (2) have them back but increase their price or add extra linen charge next year to pay for the sheets (3) have them back and put cheap sheets and mattress covers when they come back ....