Is Consentual Sex A Sin Against Human Or A Sin Against Allah?
When God has forbidden it, it it has become a direct violation to the directive of God. But why is it forbidden is the question. So we find that if one is able to avoid having children out of zina, still zina with consent is forbidden. it is actually to save the person from being lecherous, and to save the family unit. An unmarried man has no family of his own, a spouse, who is directly affected by it and the children. Therefore even if the zina before marriage apparently harm none, it is forbidden. However, If one believes that it directly harms the society despite the safety measures, one may have the other opinion as you presented.
Allah promises in Quran that he doesn’t impose any religious requirements on anyone that a person can’t handle, but restrictions on physical relationships seems to be the one that many are incapable of handling except a very few of those with solid belief and extreme discipline and control in their teen years.