In the beginning of the first episode, Alois is shown with many bruises all over his body and marks around his ankles, and he is shown laying in bed naked next to an old man who likewise unclothed.
He was "adopted" by a rich old man and raped. Suzu from the Peacemaker Kurogane manga.note Though that could be considered Single-Target Sexuality. After he gets rescued by Knives he is later seen being only attracted to and in love with men (namely Knives and possibly Vash). Legato Bluesummers from the Trigun manga is shown to have been raped (possibly numerous times) by the town mayor, or someone like a mayor.