Not my business.
"A lie by omission is still a lie" is bullshit in many circumstances, including this one.
I've told my wife about the older ladies at work I talk to. She knows where I go to bed every night (with her, if that wasn't obvious). I don't tell her every time a sweet older lady says hi to me, or if I wind up in her office and we talk for 45 minutes about nothing important. She knows it happens, and she trusts me to not cheat. And I'm not interested in cheating. I just like women in general (personality wise) and tend to get along with them. A lot of these women are married, too, and also aren't looking to cheat.
And that's just one example of the "lie by omission" fallacy.
If something isn't your business, you're not a liar by keeping someone's privacy.