Is it a sin to have obscene thoughts (no fornication involved), if you don't have the desire to fulfill them? Is it a sin to be aroused by a non-sexual image (without any sexual fantasy)?
There is a difference between temptation and lust. Temptation is when you realize you could sin and something is attracting you toward that sin. Lust is when you strongly desire something that would be sinful to do -- so strongly that you are thinking of ways that it would be acceptable to do that sin, at least in some circumstances.
Arousal just indicates that your body thinks sex may be available. When you are young, it often is mistaken. It is common for the brain to associate things with sex that really are not sexual in themselves. This is due to the way our brains learn by making seemingly random associations. It is when we reject the association that the brain learns and prunes its mistaken associations.
That is why boys are plagued with spontaneous erections when they start developing. The brain has a new function and it wants to learn when is the proper time to use it. It feels good to throw the switch, so to speak, so the brain keeps throwing the switch and asking, "Now? Oh, ... not now. Well, how about now?"
This learning process is not sinful, just annoying. It could become sinful if a guy gives credence to his random arousals and begins to convince himself that this arousal defines who he is.