Cultural relativism is the principle that someone's beliefs and activities should be understood in terms of his or her own culture. When advanced as an excuse for a sexist incident, cultural relativism tends to be conflated with moral relativism (the position that there are no objective standards of right and wrong). Apologist arguments of this type tend to take the form of "$sexist_action is common in this culture and not condemned by it; therefore, people of another culture shouldn't view it as being wrong either".
It can be helpful to compare moral judgments levied on other types of wrongs to those levied against sexism -- for instance, a given observer may condemn theft and assault regardless of cultural perspective, but offer apologetics when it comes to sexism.
Focussed criticism of a sexist action that's common in a culture does not amount to a condemnation of that entire culture; by the same token, one needs to be wary of stereotyping and othering.