I'm going to make fun of my boyfriend's penis.
That sounds very cruel. Why would you do that to him?
Because I don't love him anymore.
It's very rude to make someone, especially something that they can't change. If you make fun of his penis, it will potentially cause him to become anxious about it with future partners and ruin his confidence. If you don't love him anymore, just end the relationship rather than being this mean to him.
You don't get it.
To me, it seems like you are just being needlessly cruel because you don't love your boyfriend anymore. If there's more to the story, please share it. Making fun of your partner's body can cause long term insecurity and make them feel very self conscious.