My mom had this phrase she would say, when she finally got tired of us asking why:
"Because I'm a Mean and Rotten Mother, and proud of it!"
That certainly interjects some humor into a tense moment.
We kids often said we wanted to make a t-shirt for our mom that said "Mean and Rotten Parent" (you know, so it would be non-gender specific).
Now, as a parent, I think I need to make that shirt for myself.