Anyone who uses physical violence towards their partner is being abusive.
Someone up thread said that 'lashing out' was not the same as being an abuser, because the gravamen of domestic abuse is in the transfer of power from the abused to the abuser. 'Lashing out' is still an attempt to obtain a transfer of power, as has been exemplified by the accounts given in this thread: "I wanted to stop him calling me names" "He was coming after me and pushed a door over my foot".
If the person who has 'lashed out' is generally the abused, and not the abuser, then this power transfer is only likely to be temporary, and may be a futile attempt to equalise a power imbalance in the relationship. In a relationship that is supposedly loving, and violence free, even a small snatching of power will have an impact upon the dynamics of the relationship.
It is simply a fallacy to deny that physical violence in a relationship is not abuse. People who love and care for each other simply do not behave in a way that will cause pain or fear to their partner.
I do not think that if 'lashing out' is genuinely a one-off incident then the abused party must leave, but what must happen is for the abuser to recognise their actions as such and be prepared to work with the abused to make sure that any power imbalances in their relationship are addressed. It is not acceptable for the abuser to deny that they acted in an appropriate manner.
I do have sympathy for victims of either gender who 'lash out' after having been the abused person for a sustained period of time. However, just because they have been abused, does not also mean that they cannot be an abuser.
I do also realise how hard it is for people who have been abused (whether verbally or physically) for sustained periods of time to properly assess violence in a relationship. The trouble with justifying an abused person 'lashing out' is that it becomes harder (for the abused and for society in general) to condemn the violence of the abuser. If some violence from the abused can be justified, why not some from the abuser?
It is actually unhelpful to the person who is a victim of sustained abuse to say that it is ok for them to 'lash out' at their abuser. It is hard enough for an abused person to get their head around the wrongness of the abuse they suffer without having to deal with thoughts such as "if I can justify to myself why I just 'lashed out' at my partner- maybe there is some justification for why they hit me?"
Zero tolerance.