It's really a sexual preference lifestyle issue, but some would call it a sexual fetish or perversion. However, in many ways, male nudity permeates almost all aspects of male-to-male relationships and influences the nature of masculinity and what it means to be a man. Men use nudity and exposure of body parts or genitals to communicate affection, friendship, camaraderie, power /control, and even distain or disgust. Being aware CMNM scenes and being turned on by them is subconscious for most males. We're just a select few who have recognized it as a separate way of psycho-sexual interaction and, for some personal or social stimulation. Once you think more deeply about it, you'll be able recognize and see its effects in buddy-to-buddy friendships, sports and athletic camaraderie, male-bonding and growing up, rites of passages for young males, social events, public and private clubs and social organizations, schooling, and especially in the past - even the constant girl-chasing pack mentality of romance, male lust (and sex) and the antics and rituals associated with those.
More recent acceptance of gays in society has meant that 'guy-chasing' and the behaviors around that once secretive activity are becoming more known. So CMNM (like CFNM - "clothed female-naked male") is becoming a recognized category of human sexual activity.