Repression!
The funny bits, as you undoubtedly know, are that a) vasectomy is not castration, b) Freud's castration complex is bullshit only exceeded in bullshittyness by his concept of penis envy and c) regardless of all that, a lot of morons, almost all of them males, equate vasectomy with quasi-castration, or at least as some degree of loss of manhood.