wow, definetly didn't expect that, a bit scummy I guess however heroine was just coming onto him and he never reciprocated, they also weren't in a relationship. Technically fine but not sure what to think about this one since it seems pretty random and does not seem like he'll actually end up with his high school crush in the end. We all know this isn't gonna end with him respectfully settling the relationship with his neighbour and staying with his high school crush, he's probably gonna let them both continue to pursue him for some time like some indecisive trash or leave the high school crush after getting intimate like this.
High school crush is the most "moral" route to take now but we know he's gonna dump her for fmc, trashy guy really.