Don't worry - Pianos are out of tune as well. They don't even use even-tempered tuning, but 'stretched' tuning where the bass notes are lower than they should be, and the treble notes are a little highter (Sharper). Otherwise they just sound too dull.

Perfect tuning is based on the ratio of the Fifth to the Tonic, which was established by Pythagorus (yes, the same one) as 3:2. It is still used when you tune a guitar to itself in the common manner. But no matter how much you apply this ratio, you can never get back to an exact multiple of 2. So the event tempered scale is based on the ratio of the 12th root of 2 for each semi-tone. This makes the fifth a little flatter (or is it sharper?) than it should be, but not really enough to notice. The advantage is that it sounds exactly the same whichever key you play in.

If you use MIDI equipment, there are some software commands you can embed in MIDI files to tune the player to different kinds of scale.

Default is even-tempered, but tuning for perfect 5ths would be possible. This ONLY works in one key though, and the further away you go round the 'cycle of fifths', i.e. F-C-G-D-A-E etc. the more out of tune it will sound. You can also create scales suitable for Arabic music, for example.