Billy was raped in prison or molested
Given that Billy just got out of prison for 5 years and that scene with the gay guy looking at his penis, in which Billy really over reacts, I think it is pretty evident he had a sexually traumatic event occur.
He does not like Ricci kissing him, nor does he like to share the bath tub, and when they were in bed he curls up in the fetal position. I could go on and on.
But I posit that he is either gay, or was tampered with before.
Any thoughts?