If you do a study on yoga, you'll find it's not just a relaxation exercise, it's a spiritual exercise. Why would a christian follow the practices of false religions? There are plenty of exercises which one can do which don't involve the practices of eastern religions.
1 Corinthians 6:20-21
..for you were bought with a price. So glorify God in your body.
The issue isn’t ignorance about what yoga is. Don’t listen to this so called minister and his accusations. Its quite clear throughout old and new testament that God hates it when his people adopt pagan worship practices.