Rather than him just being too old or a teacher, those are both gross but sadly par for the course. Him being a capture target ruins his characterization as he's been presented just now: the perfect servant. Someone with that amount of discipline, yet he is a capture target for a minor, who is a student at the school he teaches in, and a noble he would potentially be working for in any other circumstances. So much for 'acing' decorum. An adult, let alone a teacher, entering a romantic relationship with a student and a minor is the opposite of good taste and propriety.