Sasha Grey -- '50 Shades' Is Bush League ... E.L. James Is a Sex Amateur

"50 Shades of Grey" author E.L. James is a hack with no REAL sex experience ... at least according to porn legend Sasha Grey, who tells TMZ, she's way more qualified to write erotic fiction than some horny middle-aged woman.

In case you didn't know, 25-year-old Grey -- who starred in 271 adult films before officially retiring at age 23 -- just released her own erotic novel called The Juliette Society.

We asked Grey if she thinks she's more qualified to write erotica than 50-year-old former "Twilight" fanfic writer E.L. James ... and her response, "HELL YES."

She also tossed in a subtle insult, calling "50 Shades" a "romance" novel ... not real erotica.