How to feed 2-state voltage into PC?
In this day and age, USB is probably the interface you want to use.

So, you're looking for something that does USB&lt;->GPIO.

A lot of FTDI eval boards do that, and clones do that, too. 

In principle, you can use your serial port idea with any USB&lt;->TTL adapter (if you add a voltage divider to bring down your 5 V to 3.3V), if that adapter has a handshake-free mode. 

Personally, the easiest way of doing this is, for me, probably getting one of the many, many ARM microcontroller tinker / eval boards. The Arduino platform is popular, but a Teensy or any ST Nucleo / Cypress Eval … board should do, too. Downside is: you'll end up writing Firmware. That's something that you'd have to get used to first, but it'd undoubtedly have many advantages if your problem involves executing some kind of logic on a hardware level.

Oh, and very oldschool: Use your control voltage to switch on or off any audible tone generator, and feed that into your sound card.