Getting an erection or leaking is out of our conscious control. It is a physiological reaction. Our bodies are hardwired to respond to any type of perceived sexual stimulation. When talking about, or thinking about one's abuse our bodies can easily respond. This happens to me also. It used to cause intense self loathing. I would remember the abuse and get an erection, so what did that logically mean other than I was a filthy disgusting sick slime bag bastard that was ok with a child being sexually abused and I was that child! The therapist told me that the brain does not differentiate between good sex and bad sex, and just sees it all as sex. So yes what took place was sex- horrible and evil sex, but sex.
There is nothing shameful about you experiencing the physical signs of arousal, it is an involuntary response. All of the shame belongs to those who abused and wounded you, not to you. Yes, it can be embarrassing, but part of being male.
For what ever it is worth women can lubricate and experience orgasm during a rape, and males can and do become erect and orgasm in the same situation. It does not mean they desired or enjoyed what took place- again an involuntary physical response.
We carry pain, shame, sorrow, and anger related to past events that we could not control.  Our bodies simply reacted the way bodies react to touch and stimulation and they will do it again, sometimes, in remembering or recounting events. 
The key point is that your erection is not always an indicator of enjoyment.  This is really confusing, because we know we should enjoy sexual touch, and we correlate that to the experience of erection.  As abuse survivors, we further correlate to unwanted sex acts.  At the same time, we can all probably remember random erections that had nothing to do with sex--I got them in algebra, and I am still NOT excited about finding my "X."
Today, I try to rejoice in the fact that I am able to experience erections, and able to feel pleasure in my body.  It is a signal to me that my body can experience goodness.  It point to a hope that I can have healthy sexual experiences.
My abuser overlayed shame on this good body of mine and the experience fills me with despair and outrage.  How many years did he take from me--ruining my perception of pleasure in my body?  In addition to physical pain and emotional pain, he poisoned my ability to enjoy sex and be comfortable in my body.
Nevertheless, I am learning to refuse his abuse "in the now."  He took something from me then, but the shadow of his abuse must not be allowed to take from me again today.  At least, that is how I think of it in my struggle.