I've had the very same problems with guys I've dated. They realize that my screenlock is password-protected, then hover behind, waiting for me to unlock it, and then reading over my shoulder at the messages and such I've got while I was away. I finally fixed that by setting up my computer around my bed; the monitor is on the bookshelf next to my pillow. In order to read over my shoulder, poke at the screen, or take over my keyboard, the person has to literally be in the bed with me. (Open season for sexual innuendo!)
I personally never ask for passwords, but they're often handed to me, and I try my hardest to forget them once their purpose has been fulfilled.