How can i tell my fiance that i want a sperm donation?

You not wanting a mixed baby is a form of racism. I know it is an ugly word and you may not want to admit it to yourself but it's true. He would be devastated if you told him you wanted a donor because you simply did not want a mixed baby. You should have thought of this when he asked you to share the rest of your life with him. Please sit him down and tell him how you are feeling before this engagement goes any further.