Just because Mashiro is inexperienced doesn't mean she's not knowledgeable... she was clearly ready to try a certain move on him right there at the end. Now the ball is in Oosagi's court to prove he's not actually uninterested in "that kind of stuff" like he says.
Unfortunately, in this kind of manga one partner has to be artificially kept uninterested to generate the comedy. If they were both horny, this could just devolve quickly into porn... although it might be an interesting writing challenge to have both protagonists be horny and willing, and yet the plot finds elaborate ways to stop them from going all the way.