Maybe this is a lost in translation problem, but what "issue" does OP have with young girls? He likes them, no? How is that an issue? Again, I see you are writing from Spain and am almost positive you meant something else in English and am just wondering what you meant.

The age difference is not that big and would shrink in relative terms when both OP and said young girls are older, but I think at issue (using the English expression "at issue" here) here is that an 18 year old is an adult and not a child. OP's contact accused OP of being a child predator. This is wrong on two counts: an 28 year old is not a child but an adult, albeit young, with almost the full scope of rights and responsibilities (save for alcohol consumption and being elected in the US), and OP is not a predator. He feels hurt from being falsely accused and doesn't know how to handle continuing (or not) communication with his accuser.

You or the accuser might have different views on relationships between somewhat olderen and somewhat younger adult women. You might find such relationships unpalatable, which is completely OK. You are well within your right to express a negative reaction. At issue here is not simply a negative reaction, but an accusation coined in terms that border on the criminal.