Handled it great, but that doesn't make it less awkward. It just is what it is: an awkward situation handled well. No need to feel awkward about feeling awkward, if you catch my drift.
I wish I had had a Dad like you are but I come to this from another angle. When I was 15 me and my dad went on a father and son YMCA canoe trip. In those days all the camp showers were open and no one worried about who showered with who. At the end of the day, my dad asked me if I wanted to shower first or him shower first even though it was communal showers. I told my dad to go first ,but after he had headed to the shower room, I decided I would go in while he was in there so we could see each other. I guess I wanted him to see that I was maturing into a man. Up until then we had been modest in front of each other, but I think if we had been more open I wouldn't have felt the need to walk in on him in the shower like that. I just walked up to the shower head next to him and without a word turned it on. He was surprised and said he thought I was going to wait. I think I embarrassed him and embarrassed us actually. I guess because the situation was exciting to me in some way, I started to chub up a little and I'm sure my dad noticed. When we were driving home, he told me it was ok if I was gay. I told him I wasn't (still not). I think if we had been more open, then I wouldn't have felt like I had to sneak to see him and see what I would look like when I got older.
Christopher (great name, btw), thanks for sharing. I hear a lot of regret in what you are writing. Just please know that you did nothing wrong. I understand how something like that can feel super embarrassing, and I know what it's like to have a dad who was also maybe overly modest about such things. Your natural curiosity and desire to bond with you dad as a young man and being proud of your maturation as a guy--all of those things are pure and innocent and good. You don't want to make you dad feel awkward or embarrassed because you love him, and that's good. But know that you yourself did nothing wrong, his feeling embarrassed by the situation was totally on him. You cannot control how another person feels. If anything, you were the mature one by not feeling that you two should treat each other any differently than you would the other guys there. The fact that you got a semi was, like you said, just part of being an adolescent and being in a new situation like that that involved things that are associated with sex. As you said, it had nothing to do with your sexual orientation, and I am so sorry that your dad even had a doubt in his mind about that. I'm sure his comment in the car to you was his way of saying that he loved you no matter what, but man, that must have felt awkward and hurtful for him to express doubt about you like that, like he didn't really know you, and the sad thing is, maybe he didn't know you as well as he should have.
I am not a father, but I know that as a son, acknowledging one's sexuality with their parents can feel awkward if one wasn't raised in an environment where that kind of thing was discussed openly and spoken of positively. There must be something in many dads where they have a hard time acknowledging that their boy (or daughter) is becoming a sexual person, an adult. They know it, they recognize it, but on some level, they would just rather keep it as an "out of sight, out of mind" kind of thing. But that is not the fault of the child, they are just following the course of nature. It is on them, and their need for growth and maturity in how they think about sexuality and their children. Again, you did nothing wrong, in fact, you did what was right; your dad should have seen that as an opportunity to be able to open up more about those matters, to affirm you as a man, and to use that as a bonding and teaching moment, like the OP did. I hope that when, Lord willing, I become a father some day, I can be that kind of Dad. As a Christian, I hold pretty conservative sexual values, but one of the values that I hold is that our sexuality and gender is good and is God's idea, therefore, we should embrace and celebrate it. We reserve sex for marriage not because it's bad or dirty, but because it is so good, and special, and bonding. There is nothing to be ashamed of for having a penis or boners or being horny; we are supposed to! We are men, and we should celebrate each other's manhood, and the womenhood of our sisters. Men can have a special, brotherly bond bec we "get" each other and understand the joys and trials of being a man, and fathers and sons share that as much as friends. If anything, fathers need to guide their sons into living out a healthy sexuality and self-image and how they treat women, etc. To pretend like our penises don't exist doesn't help that process.
Anyway, release yourself of any guilt or condemnation, forgive your dad, and seek to be the kid of man who keeps on walking in that kind of confidence--don't let this situation be a set back to you. Confidence frees other men up to walk in confidence, and that is the definition of a leader. You lead because it's the right thing to do, not bec people always like it or follow your example. The right ones will.
Thanks other Chris for the kind and inspirational words, The situation was awkward and embarrassing and there really is nothing to change about it. I had this very strange idea that I wanted my dad to see how I was maturing and developing in to a man and I guess honestly I wanted to see him too so I would know what that would look like. I'm sure my dad thought I shouldn't be getting a boner in the showers but we can't really control those things at that age. I really was embarrassed that he saw that and it ruined the whole male father and son bonding experience. I thought about the whole thing when I read the initial post. My face still turns a little red when I think about me doing that.
This just seems all too normal of a situation. We are males, its good you got to have a talk to your son about some of the biological things that will happen that are normal for him while he is becoming a man. If your LUCKY, this situation means that you and he have established that it is OK for him to talk to you about these things which he will be experiencing as he gets older. Since it happened to you it makes you more vulnerable which is good cause he knows you can be honest with him. Since you didn't have those experiences with your dad just know that is better for his development to be able to discuss these things with someone and the best person for him to talk those things through with is his own father. It's just a part of coming of age. Same for a girl and her mother.