Boys who are assaulted often get an erection or have an orgasm when they are assaulted. This is normal and shows that your body works properly. It does not mean that you wanted to be assaulted or that you enjoyed it.
Bodies will respond to stimulation, even a baby boy can have an erection the same way your mouth will water if you smell food or your knee will jerk if tapped in the right place.
Offenders know this and will, again, use it to make you feel responsible for the assaults. They need to make you feel ashamed and guilty so that you won't tell anyone.