> Also, an affair is only blackmail material in a society of prudes.

You don’t have to be a prude to think cheating on a spouse is condemnable.

In general, it also doesn’t really matter what society thinks. You can just threaten to reveal the affair to the spouse. And even in the case the spouse doesn’t really care it’s fodder they can potentially use in a divorce to get a better deal.

One thing worth pointing out is that in this case in addition to the affair with the Chinese spy he was allegedly having an affair with a subordinate, which is always problematic.