It wasn't possessive to a degree that he locks up Yukari the uke, but I liked for example how in hawaii, before they visited Yukari's parents we heard how possessive the seme MC listening to his thoughts in the car, how he hopes to create a rift between Yukaris parents and Yukari in order for Yukari to be completely dependent on him...that he is afraid that Yukari is prioritizing the parents instead of him....the writing was phenomenal in that chapter(s). BUT I ALSO loved how he's not a bad guy when it comes to his possessiveness because when the seme MC sees how much Yukari's parents mean to Yukari, he tells Yukari they should indeed go visit the parents they are important to Yukari.

"But now...Yukari might be priortizing his parents instead of me. The thought worries me. 'I love...my parents'. Its a sentiment I don't know, though....But if I take advantage of this situation...could I not make him...depend on me so much more...so that he could no longer...be independent from me?

"I'll isolate him and make it so that the only person he can depend on is me...and he'll be mine alone for the rest of eternity..." GOSH

I loved how nervous the seme MC was meeting the father and their "battle" of love over Yukari haha

Another instance we see conerning his possessiveness is when they come back from Hawaii and Hatsune is telling the seme he is glad there wasn't a rift that happened and that Yukari respects his parents a lot. And the seme replies "...A rift, huh? I wouldn't minded even if that happened. Because Yukari would choose me. But maybe this is...the best route to keep Yukari from leaving me." (Best route=Yukari being on good terms and close to his own family)

Also it seems that Hatsune's whole character trait was because he is in love with his brother in law?