Well, for starters, Passion on the Grand Line is not a Harem game and it's not a full NTR game either. It is an erotic sandbox adventure. It would seem crazy to me to try to make a good story and have great characters in this universe who are pirates and adults magically go crazy for a mc who just came of age and has to make a name for herself in the Blue Star.
Each character has their own personality and way of acting or doing, and even a past. I will always give a choice of alternate routes for people who might be annoyed that the mc is not the only one having sex in the game, but come on. Can you imagine Nami, with her personality since Oda created her, going crazy for the mc, together with Nico Robin as if by magic, living only for him? I think that those would be neither Nami nor Nico Robin, but rather two Harem Maids with Skin of them...
I will try to take control of the Thread to change some tags, as I think that was an initial mistake that was made when I got the game posted here without it being me.