Russel,
Sounds like you've hit your first roadblock on your yoga journey. I wish I could say there won't be more

When we encounter pain during our practice, it is always best to get with the teacher of your choice after class and find out a safe way you can get into both sides of the posture without injuring yourself. I have taught students with every injury you can imagine, and we can always find a variation of each pose that you can do until your body opens up a little more and is more permissive of the posture. In matters like these, it is best to think long-term. In 50 classes you will have made huge progress in all your postures and your body will have transformed, but you will still be at "day one" with pigeon pose (the pose in question). Rather than never doing it again, find a way to do it without back pain. Again, this is not good to advise over the internet, so get face to face with your teacher and post later about what solutions you discovered!
Happy journeys
Elizabeth