Luffy's girlfriend has a lot of admirers who want a go at her. And as far as luffy's concerned, he doesn't mind sharing

The night before Ace left to become a pirate, he crosses the line by taking Natasha's virginity. Sure they aren't related but they along with Sabo and Luffy dubbed themselves siblings. He recalls this memory to his closest confidant and asks for advice. Not too long after that, the Whitebeard pirates come unpon a sinking ship. Natasha's ship.

Ace crosses the line multiple times with his little sister by engaging in incestuous sexual relations with her. Only 3 years younger than him, the girl's only experience with a man is him. He thinks about her everyday since he left. So imagine his excitement when the new crew members that the Whitebeard pirates welcome is her crew

Ace catches his little sister Natasha engaging in sexual relations with Marco and he doesn't like it. Enraged, Ace decides to piss Marco off by trying to seduce her. Which works and confuses her on which man she actually wants