They just can not avoid thinking about it because their interest isn't moral it's prurient. Often it's as simple as anger over others doing openly what they either secretly desire or merely have very natural curiosities or occasional wonderings. Like reformed smokers they incessantly proselytize against the behaviors as a way to keep the ideas, and pictures, fresh in mind. By claiming their interest is faith based they can avoid uncomfortable questions about their strange concerns. Of course, if the urges become uncontrollable there are always a spare set or two of vestments to don for additional camouflage.