There is a very fine line here between protected and unprotected sex here.
I think pussy sliding as you call it, crosses that line and hence why the girls on here are not willing to do this act.
I don't believe that is always true, the lady is massaging the shaft and at no time does the penis enter the pussy so its perfectly safe, I know some ladies may get so excited and lose control and want that beautiful cock in there but that's not for me