If courgettes are bitter beyond a faint hint that normally comes with raw vegetables, don't eat them! If you are getting that extreme bitter taste it is not because your courgettes are raw. Natural compounds called cucurbitacins can collect to excess in courgettes that have grown in drought conditions or been cross pollinated with ornamental gourds. Every few years are are reports of a bad batch of seeds. It's rare (vanishingly rare in commercially grown squash and seeds) but can cause food poisoning, so dump any bitter courgettes. You certainly won't want to eat them tasting like that!