Ephebophilia is when an older adult is sexually attracted to post-pubescent teenagers or adolescents but still biologically adults—usually those in the age range 15–19. https://simple.m.wikipedia.org/wiki/Ephebophilia
I believe that ephebophilia is not morally wrong as long as it’s between two consenting adults. For example if a 18 year woman wants to have sex with me a 26 year old man then I do not think it’s morally wrong because me and her are both consenting adults so what we are doing is completely harmless.
I think that arguments of the form “X is/isn’t moral” tend to be difficult to make. And with sexual social norms, particularly so.
In general, I think that most social norms related to sex are more-or-less arbitrary.
I have a really hard time imagining a society where across-the-board murder is simply acceptable functioning. It just creates too many issues for society to function. Maybe you could have dueling or something like that, the pater familias causing certain forms of infanticide in the Roman tradition, maybe some kinds of euthanasia, but some kind of restrictions are just required to have a society. You can’t find historical civilizations where it was just okay to outright do anyone else in.
But when it comes to sex, societies have had different views on polygamy, polygany, incest (of different forms), sex below certain different ages, homosexuality, bestiality, prostitution, pretty much you name it. Maybe there were some issues that they created, but some society made it work all right.
But thing is, society teaches people a set of social norms, and I think that with sex, those tend to be axiomic. Like, someone who objects to bestiality probably isn’t going to say that they find bestiality to be wrong because it violates some other moral precept. It’s pretty unlikely that someone says “sex with sheep is problematic because it might lead to spread of prions-based disease”. They’re simply going to treat it wrong in-and-of-itself. It’s wrong because they’re taught directly that it is a social norm that it’s wrong.
If you think that something is wrong or undesirable because of some other consequence that it has, then maybe someone can try and make a case as to how it interacts with that consequence. But…you can’t really reason someone into a different set of values if the particular sex act is the root value there. If, in someone’s eyes, homosexuality is immoral because homosexuality is immoral, saying “homosexuality is moral” doesn’t do a whole lot by way of convincing them.
Maybe you can show inconsistency between two values that someone holds, and they’re put in a situation where they have to pick one or the other.