Issei's father had either taken the vows after having children or took the vows, removed them, had two kids, then took them back. His brother tried to date Taiga; it could be said that he simply didn't take it as seriously but Issei did look up to him.

Is it possible the character has an inconsistent blind spot he allows himself regarding his lovers? Particularly given three of his role models are all romantically inclined... He may be upset at the mere concept of foul temptresses, but when actually presented with it, he may not deal with the cognitive dissonance it should create. He has been positively incensed at some of his negative CR for calling him a bad monk, so it seems she plays as if Issei takes it seriously, even while not necessarily being perfect at it.