I like the idea of this fic. I would love to see where it goes. One little commend would be to describe what is happening to the characters in the lemon both physically and emotionally in more detail. For example:
You wrote: "Robin can already feel her pussy getting moist she wanted to take things further. She motioned Luffy to stop and started removing his pants, as soon as his pants were down Luffy's cock sprang out in clear view with his cock standing erect. Robin licked her lips and told Luffy to lie on the ground while getting undressed."
I might have written something like this:
Luffy's naive but surprisingly skilled ministrations of her breasts and nipples had turned her on. Robin felt her pussy moisten, drops of arousal slowing leaking from her pussy and slowly sliding down her tone, tanned inner thighs. Grasping Luffy's head she pulls him away from her prestigious bosom. Confusion, if he had done something wrong, quickly turns into excitement as Robin kneels running her hands down his chest to his waistband, pulling his pants down setting his cock free. Luffy's cock, erect from the excitement of playing with Robins breasts, smack Robin in the face finally release from its confines. Looking up through her bangs, cobalt blue eyes meet black with a smirk on her face. Robin grips Luffy's cock, giving it a couple long strokes, pulling it to the side to place a few kisses where his balls and shaft meet. Luffy groans feeling Robins soft hand touching his cock and feeling her soft warm kisses on his shaft. Lightly pushing Luffy back, Robin uses her devil fruit power to lower Luffy to the ground. Standing up, she shimmies out of her skirt; Luffy's dick pulse with blood as Robins pink pussy, dripping with arousal nestled between her soft, toned thighs enters Luffy's vision."
This is not meant to be mean or rude but just a suggestion.