kkeepers.... You bumper is welded to safely hold the weight of your bumper, maybe if you are lucky it is rated to hold a spare tire.
A bike rack is essentially a large lever. You are going to extend a lever up three feet from your bumper and then add a 90 degree load bearing arm. I am not a mathematician... but I do recall from highschool that a lever multiplies force placed upon an object. So 60lbs of bikes becomes hundreds of pounds of force applied to your bumper and that is under static conditions. Add to that the bouncing you will encounter on the highway and the force is again multiplied.
I have never witnessed a bumper being ripped off at highway speed... But I imagine it would be quite entertaining to see... Unless I was the guy behind you