Apparently the Harem Jutsu is an A-Rank Jutsu...how are Jutsu ranked? How would you do so in your fanfiction?

Apparently the Harem Jutsu is an A-Rank Jutsu...while the Reverse Harem is D-Rank (hehehe D...) but I want to know, how are Jutsu in Naruto ranked? Why is a Jutsu like Great Fire Annihilation which takes 6 or 7 water techniques to counter considered a B-Rank Jutsu when the fucking Harem Jutsu is a rank above it?!