Yours is a complicated question. From what your wrote about the incident, I would agree that you were not molested. Your dad appears to have been instructing you — period. He didn’t get off on it. He left once you understood. It was not repeated.
This is the kind of thing that often happens in groups of pubescent boys who compare their erections and show off their ability to masturbate to climax. Unless there is a power differential, it is not abuse. It is how kids sometimes make sense of their changing bodies.
That being said: You allude to current sexuality issues. Since I don’t have a way to ask follow up questions, I cannot for certain reassure you that your experience with your dad isn’t linked to the current problem in some way. However, given what you shared in your letter, I suspect that the basis of the current problem is not connected to your dad but instead is linked to other issues of insecurity or anxiety or negative self-esteem that are grounded in other experiences.
I hope you will follow up this correspondence with a few sessions with a therapist who can hear your whole story. I do believe that everyone deserves the chance to have a satisfying intimate sex life with someone they love. A therapist will help you gain insight into the source of your problem and will provide the guidance you need to address it.
I wish you well.