Define, "role-play in perfect character".After all, If the player chooses to role=play in perfect character, this would not be an issue. But, if the player disregards the character in order to take a magic sword (Excalibur), the most we, as DMs, can do, under the RAW, is exhibit the consequences to the character from the perspective of outsiders.
I will assume a few things that I figure are solidly in-genre here for the point. The noble knight wants to be chaste. He also wants to protect the kingdom from whatever threatens it this week. So, the knight has a conflict of priorities - both of which are part of his character. The point here is to ask which is dominant, and have that be an interesting choice for the player.
If we already know what is the perfect choice for this character, then there's no point in asking the question.
Mind you, this still isn't a "challenge". We are not testing whether his chastity is "strong enough". We are simply asking the player to make a decision.