When my wife told me to stop impersonating a flamingo...
When my wife told me to stop impersonating a flamingo, I had to put my foot down.
This is a pun. The joke is that to "put one's foot down" usually means to firmly reject something, but in this case it might actually mean that the jokester is literally ceasing holding up his foot. If one were impersonating a flamingo, holding up one's foot might be a reasonable way to go about it.
Of all the dad jokes on this site, this is one of the funnier ones to me.