Sure, every guy wants their sexual partner to masturbate in front of them. They are thinking that a persons hands could never replace what they have between their legs. They think, "Those are hands. You make sandwitches with hands, you drink Dew with hands."
Now a vibrator, that is something that is strictly designed to replace what a man has between his legs. A device designed to do nothing but deliver sexual pleasure. There is nothing about a vibrator that is there to put Miracle Whip on a piece of bread. And that is what scares most men.