The Greeks didn't understand asexuality and thought it was hubris, since the only examples of ace people they did accept were literal gods. Narcissus was 16 when he died. A 16 year old who constantly gets sexually harassed has the right to be a little rude in rejecting people. Ameinias, for example, asked for the 16 year old's hand in marriage multiple times. If you want to say hounding a mid-pubescent child for sex is acceptable in any cultural context, then I'm going to view you the same way I view the ancient Greeks: as a pedophile.
Narcissus literally loved no one above himself. It wasn't asexuality it was hubris in the form of solipsism and narcissism - the reason why the condition is literally named after him.
And the Greeks cerainly DID understand asexuality, go and read Hippolytus.