“Sin lies only in hurting others unnecessarily. All other "sins" are invented nonsense.”
― Robert A. Heinlein
Sex's relationship to ethics is the same as any other human activity. Nobody has car ethics, or knife morals, but these are areas of human activity where care needs to be taken to avoid hurting people. If there is exploitation, or problems with consent, Then we have ethical problems. But just the basic act of sex, by definition is not sinful, hurtful, shameful, immoral, or unethical.