Due to some lingering animistic beliefs in Thailand to do with the nature of women, it is assumed that a woman should not touch anything belonging to a monk, like his robes or bowl. This is not supported by any Vinaya rule as such.
Often laymen washing monks' bowls will scratch it, so monks prefer to do it themselves.
Kiṃkusalagavesī anuttaraṃ santivarapadaṃ pariyesamāno... (MN 26)
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