The use of pde5 doesn’t help for a lot of pfs’ers. So probably bloodflow isn’t likely the cause of pfs ED.
More likely in my opinion is a weak Pelvic floor muscle.  On internet i found this answer to the question if a weak Pelvic Floor muscle can cause ED:
The bulbocavernosus muscle of the  pelvic floor  compresses the deep dorsal vein of the penis to prevent the outflow of blood from an enlarged penis. If there is a  weakness  in the  pelvic floor and  in particular the bulbocavernosus muscle outflow of blood may not be prevented as well, leading to  erectile dysfunction
 
      
    