What does it mean that "the marriage bed is to be undefiled"?

 

https://www.dirkswart.co.za/blog/29-2-ways-in-which-sincere-couples-might-be-defiling-teir-marriage-bed

Question: What does it mean that "the marriage bed is to be undefiled"?

See Answer: http://ebible.com/answers/14413?ori=664697