Its literally up to Hinata if its rape or not.
This doesn't translate well due to this being a fictional manga and all that, but the moment she says "no" out loud. That sets the legal grounds for rape. It doesn't matter if she secretly means, "Yes" Because she audibly revoked consent, in the real world such an act would constitute as rape if victim decided to file charges. Its extremely binary but it has to be to prevent abuse through the law.
But that's only if they feel like it was rape and was actually against their consent. Even if they say "No" before or during sex, if they are actually okay with it during or afterwards, then its not rape.
There have some cases where consent was revoked after the act during to victim feeling scared of the perpetrator. Those are a bit messy to deal with.
This is why knowing your partner is important and how they react to certain things. If its a girl I knew well, I may push through her no's as I know its an act or her just being overwhelmed with sensation. But I would ABSOLUTELY NOT push on with a girl I have no experience with. You will lose that legal battle every time.
The fact that this story is this story, I don't think it will go to charges or court or anything like that. Sou honestly will want to be a father and also preggo sex and lactation play I bet. Honestly I think Hinata might get some character growth out of this, and maybe talk to Sou about her past and concerns.