The Collaborative International Dictionary of English v.0.48:

Desirableness \De*sir"a*ble*ness\, n.
   The quality of being desirable.
   [1913 Webster]

         The desirableness of the Austrian alliance. --Froude.
   [1913 Webster]

WordNet (r) 3.0 (2006):

desirableness
    n 1: the quality of being worthy of desiring [syn:
         desirability, desirableness] [ant: undesirability]
    2: attractiveness to the opposite sex [syn: sex appeal,
       desirability, desirableness, oomph]