I'm a geek, and a privacy geek, so I only surf stuff like this on my own network, which I control. All of my devices also have strong passwords (and public systems have guest accounts) and nobody gets onto my personal accounts (and my files are private). This has never seemed odd to the people I've dated because I am a privacy geek, and I don't like people messing with my user accounts on devices.
The big problem is for the folks that don't have a history of strongly separated user accounts. Suddenly demanding separate accounts would seem 'odd'. If you had to, I'd suggest blaming your SO for messing up your stuff then setting up separate accounts with passwords. (Also make sure you control the router and have it set up correctly, and know what software is running where.)
Beyond that, practice good privacy hygiene on all of your devices.