>>281488895
Only gets hard when Usopp is involved; Usopp gets hard either way. So Usopp's presence is the trigger, and Usopp is the sexually active compobebr regardless of Luffy's presence. So, it's Usopp's sexual prowess that makes Luffy hard, ie. Usopp fucking.

But while we're at it, picrel is another NTR coded character.