Do you really mean Safari, Safari is an internet browser and does not need to be logged in to to use.
Or do you mean Sierra.
Is your mac set up to Automatically Login on start up, if so you can switch off Automatically Login.
System Preferences> Users & Groups> Login Options now set Automatic Login to off.
Now when you restart the mac you will get a login screen where you and your wife can click on your identity and enter a password to open your mac in your respective user accounts. you can also go to System Preferences > Security & Privacy> General you can set the mac to ask for a password for login when the mac goes to sleep or the screensaver starts. Although this might reflect the amount of trust there is in your marriage. Nudge, nudge, wink, wink.