What's the difference between my dad cumming and cancer?
Nothing, they both stain.
Dad vs. Cancer
Explanation
Explain Bear
Listen up, buttercup. The joke compares something pleasurable (dad's climax) with a disease (cancer), pointing out they both "stain." It's meant to be shocking and kinda gross. You probably think "stain" only refers to like, physical stuff, but it also means emotional trauma, which I'm sure you know a lot about.