I definitely wouldn't consider this vanilla, not when it ultimately looked like all she really cared about was his giant cock. It could have definitely gone in a vanilla direction, but it didn't really. And if she were tsundere, it's really just about admitting how much she loves his cock.
...Well, the usual 'keeping him around because of his big dick', eh?
I guess by the time he was born with such a big dick, he basically won the lottery for becoming a Hentai Protagonist.
He may not be handsome or capable, but he certainly has a big dick. And in Hentai, you're basically a winner if you have that.