While it certain could be the case that it's defined differently from person to person, I think most people know what the range is, and a scale of only 5 points doesn't leave much room for ambiguity in my opinion. I think most people realize that a 1 would be having a partner who refuses sex, and a 5 is as good as it can get. That leaves just three grades in between, which most people could easily describe and agree on in my opinion.