1. Sanji sends Nami, Brook and Chopper away from Bege's body, saving them.
2. Sanji tells them he needs to deal with something, but will be back.
3. Luffy decides not to wait. Wants to go after him now. (You can argue that Luffy was the one that didn't believe in Sanji's word of coming back, but I find that very cheap.)
4. Sanji gets blackmailed.
5. Luffy finds Sanji after he has already been blackmailed and his spirit already was crushed.
6. Sanji decides to cut ties with the crew, in order no minimize the losses. If they interfere more and dont leave right now, they might get fucked.
7. He believes in Pudding's words. He actually thinks Pudding is the only ray of hope in this dark situation he's in. He is on the verge of losing everything he hold dear in life. His father, his ability to cook, his friends, probably also his dream of finding All Blue. AT LEAST he has someone that "loves" him.
8. He finds out Pudding is lying. His last ray of hope was now extinguished. A nice parallel was drawn here. During Baratie, Sanji was overhearing Zeff, the cooks and Luffy talking about him, how he should go with Luffy and go after his dream. They all love the soup he's made and will miss him greatly. He realizes he has a home there, with people that love him. Considering his background with the Vinsmokes, that's a MAJOR thing for him. He lights his cigar and sits there, on the other side of the door, realizing he's found something good. He joins Luffy after that and then we have the "farewell" scene. In WCI, after he hears Pudding, he once more tries to light his cigar, only to fail. This time there's isn't "hope" on the other side of the wall, there's only confirmation that he's fucked. He has NOTHING at this point....until he goes back to Luffy.
9. Despite the bento was made for Pudding, it was filled with all the favorite foods from the SH's, to show that he still thinks about them and would like to share with Pudding, who at the time was still nice, something special to him. he delivers it to Luffy.
10. Luffy confronts Sanji. He knows Sanji was not being honest when he said he wanted to leave the crew. The confronts Sanji to say what truly is in his heart. Like Nami, and Robin moments, Sanji states his true wish of going back to the Sunny, but he doesn't want to abandon the Vinsmokes, since he knows they will also be assassinated. Zeff wouldn't want Sanji to do such a thing. Luffy open up a smile and says "of course. that's how you are. Let's crash the wedding." and simply accepts him back, like he never had left.
There's 0 "pervert gag" involved here. If you hate Sanji for the pervert gag, you shouldnt consider it here. If you still think this is bad, or should be resolved with a simple "he should have trusted luffy from the start", I truly don't know what to say, except that the reasons you like One Piece are completely different from my reasons. Nothing I would say would convince you of anything, much like nothing you would say would convince me. That's because we come from completely different angles when looking at this story and how it impacts us as readers.
Having said that, Whole Cake Island is awesome. The Vinsmokes' plot, although unnecessary, was done really freaking well. It is the only post time skip arc that I enjoy as much as the pre TS ones. Being my 4th favorite one in the entire story.
Peace.