You got it the other way round. It's not if you want to keep your race pure but what the enemy thinks of your race and what steps they will take to ensure that it no longer exists.Is the purity of the race still a requirement today? I have a very strong dislike for racial purity arguments because they are implicit of racial superiority theory. Whilst comrade Xi Jinping, currently resident in Beijing, may believe in the racial superiority of Han Chinese, it makes him no better than Adolf Hitler or members of the Klu Klux Klan who asserted their racial superiority and thought that their shit didn't stink either. But it did stink and racial superiority in any form is a hateful philosophy that breeds nothing but hate and death.
Not to mention values were more conservative in the past when people believed that women that were raped were 'tainted' and had trouble getting married, especially if they had kids from the incident.
Of course that was 50 years ago and values have changed since then but as an example of what the situation was like then, you can look up the 1998 Indonesia race riots and the rapes that happened during the time when the military was involved in the incidents as well. And they did this to their own people, not their 'enemies' where anything goes. It'll be worse in a free for all war.