And I've never given my wife a single password or passcode or PIN. But she can still use my damn phone or computer if she wants, as can my friends. Worst case scenario, I get a 55 gallon drum of lube ordered on my Amazon account.

For me, the principle is I never divulge credentials, but I trust people I know. Therefore I see no conflict with adding my wife's fingerprint to TouchID, for example. (though it's not worth the effort of re-enrolling her every time I get a new device or a new screen or reinstall from scratch, so that's long lost).