Theory-the Vinsibs have a hard time with self control when it comes to their partners.
Ichiji has an urge to bite Ace's tits and it really doesn't help when Ace never really wears a shirt.
Sanji just wants to be in-between Zoro's tits but is trying to be a good and respectful boyfriend to Zoro.
Yonji gets turned on hard when Sabo goes apeshit, but tries to hide it.
Niji wants Yamato to pin him because he likes it when Yamato is the Dom in the relationship, but he doesn't want to move things too fast.
Reiju admirers Tashi ( I don't know how to spell her name) on how brave and protective she is and just wants to get in bed with her already, but like with Niji she doesn't want to move things too fast.
(I feel like, first, I gotta explain that this is a Headcanon, not a Theory, and it being a Headcanon doesn't make it any less valid, in general, it's just a different thing).
I subscribe to this! 100%
And if I may mix in my own headcanons too:
Ichiji is over here seething about how horny Ace has made him to be; he, along with Reiju, is the least horny of the siblings, however, ever since Ace came into his life, his level of horny has increased, and it doesn't even matter if Ace is wearing a shirt or not, and he's usually not! He, however, likes boobs as much as the next Vinsmoke and Ace all has his out for the world (Ichiji) to see and it makes his hands itch to grab and he needs to bite himself or he'll end up drooling with want to suckle and bite, and isn't that embarrassing? (Ace doesn't think so but Ichiji is dying inside).
Sanji, known pervert, is having nosebleeds trying to stop himself from jumping his boyfriend and diving into those wonderful tits, Zoro is just trying to see for how long can Sanji last (he's having fun) while tyring to be tempting from time to time.
Whenever Sabo fights, threatens and/or goes completely apeshit (and maybe ends up covered in blood or sth), Yonji squirms, doing his best to not go crazy (hearts in his eyes, tongue out, doing flips, not unlike Sanji does), but it's a losing battle, he always ends up cheering for Sabo with hearts coming out of his eyes and his dog tail wagging so strongly he either flies away or dislocates it (Sabo thinks it's really cute).
Niji refuses to ask Yamato to put him in his place but he still tries from time to time to test his patience to see 1) where's the limit and 2) maybe also get Yamato to flex him or something (Yamato usually picks him by the scruff, like a kitten, which works on Niji, and that annoys him but he can't do anything about it); Yamato doesn't realise that's what Niji needs, so Niji will need to abandon his pride and ask (sooner or later, preferably later, in his opinion).
And Reiju has fun teasing Tashigi (teasing and being seductive, she's good at that) but she's not pushing many buttons, Tashigi is cute and cool, so she wants to take her time and enjoy the more innocent part of the relationship, but sometimes Tashigi (accidentally, with all her sincerity) has Reiju having to look away and stop herself from having a nosebleed and jumping her girlfriend (she succeeds, among the Vinsibs, she's the best as succeeding).
(and please feel free to share more headcanons!)
