At least in part, it may have to do with clerical celibacy. If one truly observes Church teaching, a young man, unmarried and so a virgin, enters the seminary and embarks upon a life allowing no physical expression of love (or lust, for that matter). Since our natural sex drive is so powerful (and younger men get erections they weren't even trying for)it must be a constant struggle. In a conversation with a much younger woman friend of mine (she works full-time, has two little kids and a wonderful husband who works two jobs.) I said to her, RE: this topic, that it seemed to me that the clergy think the laypeople are having way too much fun in bed and they'd like to put a stop to it, and she said, wryly and truthfully, "Sex? Are you kidding me? I'd settle for 15 minutes of uninterrupted conversation with my husband."