I say, with Allah's guidance: Washing is obligatory after penetration in the anus for both the doer and the one done to, despite its prohibition. If the two circumcised parts meet and the glans is concealed in either of the two passages, washing becomes obligatory whether ejaculation occurs or not. Ali, may Allah be pleased with him, said: 'You impose the penalty for it but do not require a measure of water for it.' See: Al-Wiqaya p.95, Fath Bab Al-Inaya p.321, and Al-Sa'aya p.321. And Allah knows best.