He needs to turn her "No" into a "Yes" using the mightiest weapon know in the hentai world: the dick of a hentai protagonist! The first step is already made as a no with a heart is not the same as an ordinary no....
Loved her facial expressions in this one. And my mans....don't tie yourself down. She said it herself, you were in the right place at the right time. She ain't your girlfriend so it ain't none yo business if she want to screw the janitor. Find you a girl who want's only you because it is you.
That ending...
Well, preventing her from going to other guys and 'trying' other dicks is an accomplishment in and of itself, so...Just keep trying.
I'm sure with enough persistence and lots of sex, you'll eventually convince her.
"...yeah, alright."