What kind of touching necessitates ablution according to the Shafi'i school?

Touching one's wife or a non-mahram woman who is desired nullifies ablution (wudu') according to the Shafi'i school, absolutely. The reality of the touch that nullifies ablution is the meeting of the two skins, even if it is slight. This includes touching intentionally or forgetfully, with desire or without it. However, touching a tooth, a nail, or hair does not nullify ablution according to them. There are two opinions among them regarding the nullification of the ablution of the person who is touched, and most of them have authenticated that it is nullified. And there is no blame upon whoever adopts the view that ablution is not nullified merely by touching.