You are absolutely right that it is a separate issue. There is no question about it! I can always scan a picture of Fann Wong and put her face on a pronography site. I do not even need to do photography.
And perhaps that is one of the reasons (there are thousands of reasons why people do not like people to take their pictures - particularly strangers!) why ladies and even man may loathe their pictures taken.
However, when a photographer approaches a person to request to photograph her/him, the "subject" can at least "assess" the intent of the photographer. Sure this is not foolproof, but at least the "subject" feels that the photog respects him/her.