I'm not good with explanation but I think he wanted to piss off his father by officially make him his wife. Their marriage was arranged for political reason and his father wasn't exactly keen on accepting a fox as in law. That old wolf fully intended only locking up hinata in prison but the seme went against that.
Read the story again it's explained ╮(￣▽￣)╭
Uh..he said it was like a vow they make. Yk like they said you'll become a true married couple once you consummate? Though he did indeed force himself on hinata(and he admitted to this) because he was running away, he wanted hinata and himself to be a couple and the wolf village won't accept them as a couple unless they do it.
He did the act to save him basically ٩(๑❛ᴗ❛๑)۶
Yeah the main reason being this! Saving hinata. Making his pretty wife happy. ヾ(❀╹◡╹)ﾉ~
I'm rereading this and the Seme legit raped the uke with a deranged look on his face and then did a complete 180. Did I miss where it was explained why that happened?