Pure water won't do any more harm to wood window frames than rain water. If the frames are in poor condition, ie, gaps between the glass and the putty or seals, or cracks in the wood or joints where the sun has caused expansion, then water will get in - any water.
If the frames are maintained in good condition with regular painting or varnishing, sealing any gaps or cracks, then water - pure or any other kind - won't harm them.