Setting restrictions in contest rules is unarguably fair. If the rules had said "no use of the colour orange" - well, that's the way they are, and an artist should be able to rise to the challenge. The only objection is to the implication that nudity is "dirty", which presumably is the personal opinion of the person writing the rules, and who is accountable for that remark. That may be their personal opinion, but it was insensitive to air it in this manner. As the anti-nudism campaigner remarked, "If God had meant us to run around without any clothes on, we'd have been born naked!"