I Hated It (I Think)

I'm not the most masc guy. I spent 2 weeks in a county jail earlier this year. Overcrowded, 4 men to a cell, with 2 sleeping on mats on the floor. A few days in my cellmates made it clear they wanted me to sexually service them. I said no. Next day I "slipped in the shower". After a few more bruises I gave in.
Guards did a walk around and count every 30 minutes. That night the guard walked by. When he passed one of my cellmates got out of his bunk. I pulled my jumpsuit down to my knees and laid on my mat face down. He rubbed hand lotion in my asshole then got on top of me. I took a guys cock in my ass for the first time. It hurt like hell and I buried my face in the pillow and cried. He finished. I covered with my blanket until the guard walked past again. Then one of the other guys got up and took his turn. All 3 of them fucked my asshole that night. The next day I was sore and walked around all day feeling wet from the cum leaking out of me.
I didn't say anything, I was too embarrassed. After a few days of the same nightly treatment I got used to it. It didn't hurt nearly as bad. But something else happened even worse than the pain. In the middle of being fucked I felt something uncomfortable. I realized my own cock was hard and it was digging into the mat as he was on top of me. WHY WAS I HARD? I didn't enjoy being r**ed. But it happened several more times before I was released. I was humiliated, hurt and abused but why did I get hard?