How CN I tell if it damaged me or not
Well, this leads me to wonder why you are asking about it? If it didn't damage you, and was a one time event you probably wouldn't be bringing it up. If you weren't having some kind of struggle, you probably wouldn't be on a PTSD forum.
The problem with humans is we aren't absolutes and we are all different. Trying to find a clear-cut, black and white answer to your question is not going to work. What if we gave you a list of things that would show it damaged you? You could look at the list and find that most of the things on it applied to you and still not have the proof that what happened damaged you. You could find ways to explain away everything on the list. Anything on the list could have also been caused by other things. None of us, not even a therapist, can look inside your brain and say this symptom was caused by that event and only that event.
I spent years (decades maybe) looking for proof. I wanted something that I could point to that conclusively proved that the things my dad damaged me. That what I suspected was true. Evidenced mounted up, but it still wasn't the magical proof I was searching for. It just lead to more questions, different doubts, or resistance. The rotten thing is, we can't answer your question. You are going to have to figure it out. Asking the question is a good start, but you are also going to have to examine your feelings and memories and look at how you dealt with things then and how you deal with things now. In other words, it's a whole lot of work. It's also learning to trust your memories, feelings and experiences.
How can you tell if you are damaged?
You feel or believe a past event damaged you
You think about a past event and wonder if it damaged you
You remember a past event often
When you remember a past event it causes you anxiety, intense emotions
You have nightmares about a past event
You have shame around a past event
In your case, you have issues with masturbation
You think about if it happened to a child you know and believe it would harm that child
The list could go on. The things on this list, also may not fit. For example, people can remember positive events often. If you are grieving you can have intense emotions about a past event and that doesn't mean you are damaged. That's what makes it so hard. That's why at some point, you have to stop analyzing. What I did at that point, was decided to move forward "As if" it had damaged me. I could still think there was a good chance that I was misinterpreting what happened with my dad, but I was going to act as if it was real and see if it brought me healing. When it did begin to bring healing, that was further information, that damage had been done.
I hope you find answers and healing.