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"Leaving the law aside and dealing only with what is right and what is wrong (because morality and laws don't always match), why would it not be okay for ole Max to have girls 16 and 17 in his films..."
You're exactly right. Eighteen is, in fact, an arbitrary line. It was drawn by temperance advocates in the 1880s in response to a sensationalized, and largely fictional, white slavery scare. Prior to that, it varied from state to state, but was usually somewhere between 12 and 16. Personally, I think that 16 is, biologically and psychologically, closer to the mark of actual adulthood than 18.
However, 18 is the line that has been drawn, and the woman in question was, in fact, over that line. Thus she was, according to both the law and her own testimony, a consenting adult.