I can understand squeamishness about organs of excretion - but are feet less auspicious? Didn't Krishna wash Sudama's feet?
Yes as per the scriptures. The part of the body which is above the navel is considered as pure but not so the part that lies below.
Manu Smriti 1.92. Man is stated to be purer above the navel (than below); hence the Self-existent (Svayambhu) has declared the purest (part) of him (to be) his mouth.
Similarly, from the Dharma Sutras of Baudhayana (BDS):
After tying or untying the lower garment, he should wash himself or else touch wet grass, cowdung, or the earth. When he is engaged in ritual activity, he should avoid touching below the navel, for a vedic text states: ‘A man’s body is clean above the navel and unclean below the navel’
And the Vedic text it is referring to is the Taitiriya Samhita 188.8.131.52 (Krishna Yajur Veda).