Purity: Ghusl after Forbidden Intercourse
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 350:
If, God forbid, a man has sexual intercourse with an animal and the man ejaculates, ghusl is sufficient. If he does not ejaculate and he had wuḍūʾ before penetration, again ghusl is sufficient; but if he did not have wuḍūʾ, the obligatory precaution is that he must perform ghusl and wuḍūʾ. The ruling is the same in the case of a man having sexual intercourse with another man or with a boy [God forbid].
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)