most likely they didn't had this conversation when they were younger
but they had it in game recently (I'm pretty sure) anyway the point is if that were the case she already knows that MC does not mind if Hiroko wakes up in the middle of the night and mounts him , so he wouldn't be bothered by her actions, therefore he wouldn't be upset if something like that happened and caused Mao's birth,
therefore he would have told him about that , that's why I don't think it's what you're saying
even if you add to your theory that she thinks MC will be upset because she hid this from him for 18years and could have easily been solved , even then I still think it's a stretch