That's why you're in a SB relationship. No matter how much you and the girl get along it is still a transactional relationship. If one of you looses interest and isn't keeping up with the deal then you move on. PPM is the way to go.My only concern is a common trend, it seems , where the cash flow is steady but the interest to keep the sex declines over time. And that can happen anytime - thus the PPM.
And yes to the single mom dynamic. For her it's a chance to have a bit of fun away from the responsibilities of family and get some extra income at the same time. When I resume the search of another SB that will be my focus.