Prior to Magneto mutants were more rumors than anything. There were some sights sure, but there were few and far between. maybe some people were nervous but that's it.oh really? how were human/mutant relations prior to Magneto's debut? you don't think Mystique assassinating Graydon Creed contributed? or how about Stryfe killing Charles Xavier? or the Weapon X program?
Then Magento shows up starts launching very public attacks while giving speeches about how mutants are destined to rule and replace humanity and issuing commands to a group with EVIL right there in the name. First impressions are really hard for people to get past and Magneto set the tone fro mutant and human interacts from then on.