Of course, it is clear that it is the act not the person which is 'detestable'. In fact, some modern translations make that clear by translating the word "towebah" as a "detestable act" in this instance
But that begs the question you are implying. A 'priest' in the RCC is an influential person (much as a teacher in the larger society would be). And, assuming arguendo that the act is detestably immoral, then one who is inclined toward such behavior (even if he is not then practicing it) would be disqualified from that leadership role until he renounced the practice -- not just for himself but for those under his leadership.
In such an instance, he is not disqualified by his sinful act (assuming again arguendo that he has refrained from the wrongful act) but by his inclination to such sinful acts in a leadership position.
This is more than academic. With so many broken homes and accordingly so many teenage boys coming to manhood without the inlfuence of a male, sexual confusion puts more teenage boys at the margin and increases the danger of wrongful influences in positions of leadership.
That is why our society must be vigilant about homosexuals (whether practicing or of inclination) in leadership roles near young boys coming to adulthood.