Sorry, lots of people in here prefer to talk about rabbit sex.
The thing with Jinbe here is that he's generally pretty honorable. Not to a fault, not in a way where he'd say "well, I guess I'll take my comeuppance and spin this wheel," he's not an idiot. He knows what will happen there. But at the same time, out of deference for being taken under Big Mom's wing, he feels it's his duty to properly resign. Or at least to let her know it's his intention.
Sure, he "pulled a pirate" and just did whatever he wanted in the end, but it's a bit of a struggle for him I think to find the line between being honorable and a scallywag. Certainly the appropriate place isn't "let her cut off my leg" though.