All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Touching the buttocks does not invalidate ablution but if someone touches his penis, whether voluntarily or involuntarily, his ablution is invalidated. For more benefit please refer to Fatwa 85221.
Allaah Knows best.