@Echonic It has literally nothing to do with "thinking with your dick".
Or maybe it does, in a certain sense, which is irrelevant, because the reality of it is that experiencing sexual arousal when subjected to a certain kind of titillation (visual stimulation, teasing, etc) is just the basic norm of healthy human physiology.
I'm not saying he should drop his pants right there and start an orgy. If anything in context he may have every right to be guarded. Or even to just not be well disposed for sex, period.
But this recurring theme so common in anime and manga that you are scum (or at least "pervy") if you have any sort of physical response to teasing is just pathetic.
If you are a man and your body and mind aren't even remotely titillated by a swarm of girls being sexually allusive around you, you aren't proving to be a "kind and decent guy" but to have some concerningly low levels of libido and probably some hormonal dysfunction.