I am going to make the assumption that you are not traveling with a married couple but with two friends, also female, who happen to be married.

What about the state of marriage sets them up to share the bed and thus be in a position to make the claim that they are entitled to share half the hotel bill?

I would suggest that who sleeps where is either picked by lot every night or a rotation of who gets the single. The recommendation that you get a room of your own is a good one too but it is too bad that everyone has to pay significantly more.

Reminds me of dinners when my dining companions have drinks, wine and the steak and lobster while I had water and an appetizer. Of course they want to split the check by the number of people.