If a fly lands on hand towels or other clothes and then touches hand, mouth, or food itself, is that a problem? Is that like eating a bug to eat food that touched a surface where a bug touched?
Generally it is not a problem because there is nothing forbidden about a place that fly touched. If the fly however layed eggs on the towel, they should not be eaten, which is not a commen occurance. There are times when certain meat flies can land on proteins and lay eggs on them, the eggs would be an issue, but this doesn’t happen with towels.