The Collaborative International Dictionary of English v.0.48:

demasculinize \demasculinize\ v. t.
   to remove the testicles of a male animal.

   Syn: emasculate, castrate.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

demasculinize
    v 1: remove the testicles of a male animal [syn: emasculate,
         castrate, demasculinize, demasculinise]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "demasculinize":
   castrate, desex, desexualize, devitalize, effeminate, effeminatize,
   effeminize, emasculate, enervate, etiolate, exhaust, geld, sissify,
   soften, sterilize, unman, unnerve, womanize