I made it to chapter five before I gave up.
While the premise of the story is interesting, you've moved too fast for it to remain so.
There is no character growth and the characters are so out there you might as well give them original names and make this original fiction.
The girls or Harry for that matter would not be so open with sex right off the bat.
Petunia lusting after Harry is just wrong on so many levels.
Harry has a big penis; we get it, you don't need to mention the size of it repeatedly nor do you need to put in the centimetres and inches.
The permanent erection is also highly improbable, if it were so Harry would be in a lot of agonising pain.
I'm assuming English isn't your first language and while I've seen worse, you definitely need to get someone to read your work before you post it. The grammar and your sentence structure leave a lot to be desired. I'm not perfect at it, but at least what I write is comprehensible. The way some of your sentences are strung together make little to no sense.
With a bit of cleaning up this can be a good read. I hope that the criticism given helps you, it was given in that vein, and not meant as a flame.
Good Luck!