It doesn't matter how careful one is when detailing their car. Everything, and I do mean EVERYTHING induces marring to some extent. Don't believe me? Try purchasing a 1,000,000 candle utility flashlight and shine it on a dark colored car that looks "perfect" under normal lighting (even direct sunlight, asi my wife's black 330I).
As for MF towels not causing any swirls, the marring is too small to see immediately, but over time will become evident. It's not the MF that's causing most of the marring, but the 'invisible' dust that almost immediately settles on your car...even when detailing indoors. That's why everything should be applied, washed, buffed in straight front to back motions.....this will minimize the visibility of the 'build-up' of swirls that you induce every-time you touch your car.