"neglected to discover during our courtship that I had two left feet.
Strange thing is, my Grandad on Mum's side of the family was a Morris dancer and Dad was a really good Ballroom dancer. I don't seem to have inherited the gift from anyone."

Aha! Mystery solved.

You have one Morris dancing left foot from your Grandad on Mum's side of the family, and one Ballroom dancing left foot from your Dad...

:-)