Yeah, I think that assumption was the weakest area of an otherwise provoking, if not entirely compelling, argument. It would be stronger in my opinion if you were to show how the "mature" portrayal of sex directly affected the acceptance of film as an art form. Providing examples like "The Graduate" is a good start, but correlation does not imply causation.
That said, if you had mentioned this project beforehand, I would've recommended talking shortly about Japanese eroge dating sims and visual novels. These games often focus very strongly on the intimacy aspect, which ends up making the eventual sex scenes more meaningful and rewarding. Then again, eroge are not generally considered works of art, so perhaps that wouldn't be so helpful to your argument, heh.