Is the principle mentioned by the man correct: that if an act incurs no worldly or otherworldly punishment, and there is no explicit text prohibiting it, then it is reprehensible (makruh) and not forbidden (haram)? And is every act for which there is no evidence indicating its desirability considered obligatory (fard) by the scholars?

We advise you not to argue about what you do not fully comprehend, lest you fall into speaking about Allah without knowledge, which is forbidden. Allah Almighty says: "Say, 'My Lord has forbidden immoralities, what is apparent of them and what is concealed, and sin, and oppression without right, and that you associate with Allah that for which He has not sent down authority, and that you say about Allah what you do not know.'" As for distinguishing between major and minor sins, some scholars hold that a major sin is one for which a warning, curse, or wrath has been mentioned, and that a minor sin, if persisted in, turns into a major one.