From Apastamba Dharma Sutras 22.214.171.124:
A [student] who has broken the vow of chastity, shall offer to Nirṛiti an ass, according to the manner of the Pāka-yajña-rites.
This same Prayaschitta is also found in Taittiriya Aranyaka 2:18, Manusmriti 11:119, and Yajñavalkya Smriti 3:280.
How come the Brahmachari has to sacrifice a donkey to atone for this sin?
And also, how does this Prayaschitta even apply in modern times, since Vedic animal sacrifices haven't been done in hundreds of years?
Then, if this prayaschitta can't be done, the Brahmachari loses his caste (and becomes a Shudra), because breaking a vow is cause of loss of caste:
Defiling a damsel, usury, breaking a vow, selling a tank, a garden, one's wife, or child, ...... are all minor offenses causing loss of caste (Upapataka).
So does one who breaks the vow of Brahmachari have to do two prayaschittas? One for breaking the vow of chastity, and one for regaining the caste on losing it due to breaking the vow of chastity?