My sin in connection with my predisposition to obesity would be to overeat to the extent that I would harm my body. You might know that you have a predisposition to obesity by looking at your blood relatives. If you knew that your grandfather and father were alcoholics, it might be a sin for you to drink any alcohol at all. The sin of alcoholism is the love of something more than God and than God's gifts of marriage, children, gainful employment, responsibility in caring for body, etc. These things make up the sin of alcoholism.
Alcoholism has the characteristics of a disease.
Fifth Commandment and God's First Commandment among others. A non-practicing, repentant believer in Jesus Christ who is addicted to alcohol has the disease of alcoholism, but has his or her sin forgiven. The practicing, impenitent, non-believer in Jesus Christ has both the disease of alcoholism and the sin. You inquired about the position of Lutheranism regarding its followers practicing social nudism. Although we cannot give an answer for all of Lutheranism, I can give you my answer as I think about your question from a Scriptural position.