‘The concern for me about any person who did this would be would they do it to a live animal? Is this an indicator of possible, future harmful behaviour?’
In the scenario nobody else knows about it so whether it is an indicator wouldn’t matter. The moral questions would be if carrying out the act had an impact on the person that increased the likelihood of them, and if acting that way was harmful to that person’s mind in the moment they committed it. Adherents of traditional morality would answer yes to both.
‘But we assume that morals are rational, we should be able to philosophise, reason our way through to morals. Doesn't seem to have room for instinctive disgust.’
Well yes exactly. Traditional morality gives reasons why it is wrong. For example sex should be about being open to life, which sex with a dead chicken is not. Or sex should be about loving relationships, which sex with a dead chicken is not.
It is only people who have rejected traditional morality and rely on fewer moral principles who are left grappling with attempting to justify their disgust.