The Collaborative International Dictionary of English v.0.48:

Misalliance \Mis`al*li"ance\, n. [F. m['e]salliance.]
   A marriage with a person of inferior rank or social station;
   an improper alliance; a mesalliance.
   [1913 Webster]

         A Leigh had made a misalliance, and blushed
         A Howard should know it.                 --Mrs.
                                                  Browning.
   [1913 Webster]

WordNet (r) 3.0 (2006):

misalliance
    n 1: an unsuitable alliance (especially with regard to marriage)

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "misalliance":
   a world-without-end bargain, abnormality, alliance, anomaly, bed,
   bond of matrimony, bridebed, cohabitation, conjugal bond,
   conjugal knot, coverture, holy matrimony, holy wedlock,
   husbandhood, ill-assorted marriage, impropriety, inadmissibility,
   inapplicability, inappositeness, inappropriateness, inaptitude,
   inaptness, infelicity, intermarriage, interracial marriage,
   invalid linking, irrelevance, irrelevancy, maladjustment, marriage,
   marriage bed, marriage sacrament, match, matrimonial union,
   matrimony, mesalliance, misapplicability, misapplication,
   miscegenation, misconnection, misjoinder, misjoining, mismatch,
   misreference, misrelation, mixed marriage, nuptial bond,
   sacrament of matrimony, spousehood, uncongeniality, unfitness,
   union, unsuitability, wedded bliss, wedded state, weddedness,
   wedding knot, wedlock, wifehood