The first obvious question is left or right-handed guitar? It is not necessarily the case that a left-handed person plays a left-handed guitar or a right-handed person plays a right-handed guitar.
The best way to refer to this is a right-handed guitar should be on the left leg and a left-handed guitar on the right leg. However, it is good to know why and what difficulties you will face if you don’t follow this guide rather than dictate this.
Your Back – Your Shoulders are angled when sitting with the guitar on the wrong leg leading to back pain and posture issues. When on the correct leg, the shoulders are level.
Standing up to play – If you ever want to perform standing up, the guitar will be in a different place to your hands if you play seated with it on the wrong leg. The difference in hand position leads to more mistakes. On the other hand, if you sit down from standing, the guitar naturally falls into place on the correct leg.
The angle of the neck – The guitar neck is angled upwards when on the correct leg and is less strain on your fretting hand wrist. The guitar neck usually angles downwards on the wrong leg, encouraging more wrist bend and strain.
The fretting arm on leg – Due to the angle of the neck, the fretting arm sits on the leg. When on the correct leg, the arm cannot rest on the leg and has the freedom to move.
Neck Access – On the correct leg, your body doesn’t stop access to the upper areas of the neck.
Finger Stretch – Instantly, you have more finger stretch available for those problematic chords and licks due to all the posture corrections above.
And finally, I agree, it doesn’t feel natural to have your guitar on the correct leg at first. But, with a bit of effort, you quickly get used to it and notice the difference in your playing.