5" is pretty close to "normal"...whatever that means.
Just as men have different preferences about the size of women's breasts, women have different preferences about the size of their lover's penises.
I think you are wrong about all women wanting 8" penises. I think few would really enjoy have intercourse with someone with a long penis that kept pressing against their cervix.
Stop worrying about how long your penis is (not that there is anything you can do about it anyway) and become the most caring lover you can be. Believe me, most women would much rather a kind, caring lover who makes sure their needs are satisfied manually, orally or via intercourse than have a lover with a large penis who could care less about their pleasure.