I'm going to serve a guest a mug with tea stains
Do you want this guest to think you can't clean properly or something?
It will be a great joke and they will enjoy the humor in it.
That is a disgusting thing for you to do. It's bad to serve guests with dirty dishes. I don't think they will find it funny to have to drink out of something dirty.
The older guests know that it's a stupid joke and they find it fun, but the young people think it's gross and they don't know it's a joke.
I think anyone would find it repulsive to eat or drink out of dirty dishes.