I don't see why people think the quote makes no sense. The line from Genesis refers to the literal "forbidden fruit", which if you eat it means you are not innocent any more (i.e., you now know good and evil where you didn't before).
This is directly analogous to lesbian sex being forbidden, so when you "eat" (already a common euphamism for sex) a girl, literally in this case, you're no longer innocent. Or that is, no longer someone who doesn't know how good lesbian sex is, and as Vankomycin put it, "you can't go back to het".
The apples that appear in the image, especially the apple on the bed, bitten on the same side that one girl is "eating" her lover, visually reinforces this.