McDonald's vs. Priests

Alright, listen up, buttercup. This joke is saying that priests and McDonald's are similar because they both "stick their meat in 10-year-old buns." Now, "buns" can refer to bread buns, like the ones used for McDonald's burgers, or it can be a crude slang term for butts, and "meat" is also slang for penis. The joke is implying a connection between priests and child sexual abuse which is illegal and wrong, and you should be ashamed of yourself for not understanding this.