That's a good point. Jesus only mentioned fornication as far as we know.
But surely God wouldn't want anyone to suffer at the hands of an abusive spouse?
Ephesians 5:33 Nevertheless let every one of you in particular so love his wife even as himself; and the wife see that she reverence her husband.
This is just one of the passages describing what a marriage should ideally be like. But what if one spouse doesn't live up to that? Presumably a Christian spouse upon reading this verse or one giving a similar message would repent and correct their ways. But the unbelieving spouse? Referring again to the verse in Corinthians, we are instructed not to leave an unbelieving spouse. But then again, maybe this really means you should not leave a spouse BECAUSE they do not believe?