It's obviously not irrelevant for two reasons, one practical one moral. The practical one is that you don't need to 'spill' secrets to a spy. Having one around you in private means you can be bugged, blackmailed, have your means of communication compromised and so on. It makes you vulnerable by definition.

Secondly if he's willing to lie to his wife and family he's obviously willing to lie to anyone for much less. It tells you something about the character of a person, how can anyone trust a cheater?