到底勃起是不是使用了槓桿原理?

it is not.

lever system is you have a hard (or better describe as "rigid") rod, apply force to somewhere of this rod to do some kind of "work" like lifting weight or "amplify" the force.

so look at your organ again, before erection it is not any "solid" or "rigid" object. it stand up just because blood go into there and it become "turgid" 膨脹, like what the upstair said, blowing a balloon

你想像一下用一個氣球(它是未充氣的)再用水狂放進去.佢就會漲起.勃起因為你興奮.血液充滿到你的下蔭.所以就係咁lo~