Why is it considered morally wrong for a man or a woman to have a romantic or sexual relationship with someone significantly younger than themselves?
If the case does concern minors, as it might, then the question is still an interesting one. Why do we arbitrarily cut people off from certain sorts of behaviour at a certain age? We know that many of them might be more mature in character than those of legal age, and that many of legal age are incapable of entering into a relationship in an appropriate way. I suppose the answer is that any cut off age is going to be arbitrary and that the state has to legislate in such a way as to put most people into plausible categories. Most minors would be thought to be unable to know when they are being exploited or to have too little experience to know what the character of a particular relationship really is. This holds even for those who are not minors but still rather young.