I've been thinking a lot of about being tempted by seeing things and whether that's a sin. Everybody else changes their clothes and takes showers with other guys and don't think about it all. Temptation is not a sin, but I'm scared and ashamed about what is tempting for me. Not because I do anything wrong but because I think some of the other guys are so beautiful -- not their hearts and souls but other stuff.
What I learn from you is that I shouldn't worry about the temptation but about what I think about later because I can control that. And I guess what you're saying is that it's OK that I touch my private parts as long as I'm thinking about healthy relationships. I've never actually masturbated, only rubbed myself through my underwear. But I have trouble thinking about healthy relationships, like marriage and children, instead of unhealthy acts.
Let's back up a bit. It is fortunate for most of us that other people can't read our thoughts. But we often assume what other people are thinking based on what we see. You know you are having a hard time keeping your thoughts under control in situations where there are guys in various states of undress. You are assuming that you are the only one having this problem. I can definitely assure you that it isn't the case at all. Since you are a late bloomer, many of the boys around you have had more years of practice keeping control over their reactions, but it doesn't mean that they didn't have problems in the past or that they aren't struggling but just managing their reactions better.
Your admiration for more matured bodies is understandable. Those boys are where you would like your own body to be in its development. Now that your body is able to respond to sexual arousal, it is easier for you to imagine what having a more mature body would be like. But there is danger here has well. Your brain is trying to figure out when is the appropriate time to react. It starts out overly broad and then trims down its responses. Thus, it is common for young men to get aroused by being around other young men who are unclothed. It is not because they are men. It is the novelty of seeing other people fully when you yourself are now aware of the potential of sex. Yet Satan uses this as an opportunity to lure young men down a path where they are persuaded that they might be homosexual. The true fact is that your body has simply gone wild with new abilities and anything with the remotest sexual connotation (and many things which have nothing to do with sex) will set the body off.
So let's put things back into perspective. We all like beautiful things. There isn't anything wrong to say a girl is pretty or a boy is handsome. There isn't anything wrong to say that a boy is in good shape or that you hope in a few years to build up to something like that. It becomes wrong when you put those thoughts into a sexual connotation. That is why Jesus said, "But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart" (Matthew 5:28). The fact that you are being tempted from directions you never thought possible is a simple result of you being new to sexual feelings. There is a reason Paul told Timothy, "Flee also youthful lusts" (II Timothy 2:22). Satan attacks at the weakest point and since you don't have any experience yet in handling sexual urges, it becomes an all out attack in the area of sex.
You are right, you can't stop Satan from tempting you, but you can battle him over consideration of what he is offering.
Another thing you need to be careful about at this time in your life is the trap of obsession. The teenage boy's mind because it is making radical changes in preparation for adulthood is easily trapped by compulsive behavior. Since rubbing your penis feeds pleasure back to your body, it is very easy to get compulsive about it. Since at the moment there isn't a need because you aren't producing semen yet, you would be better off limiting doing such. I'm not saying it is wrong if you are keeping your thoughts under control, I just saying it isn't the best use of your time at the moment. It is hard enough keeping your thoughts controlled without getting yourself sexually aroused and then still try to keep your thoughts in line.