It's complex.
It's a defence that refers to the victim's incapacity. We can think of cases of dementia, for example. It's rather marginal. It's a defence that could easily be used by people who are violent towards their partner while they are vulnerable, precisely to evade enforcement of the law.
An alternative solution would be to remove these two paragraphs. There's no need to insert a defence right in the text. If we decide to keep this text, we should state that it is up to the accused to show that his conduct was reasonable in the circumstances.
That is my answer to your question.