This response is from a rather superficial and explanation understanding but it is where my understanding lies. From what I understand, there are a number of mechanisms involved with preventing an immune response and a couple of the mechanisms is explained by the IgA protease found in the sperm fraction and the Hu-FEDS hypothesis.
IgA protease will act to prevent IgA, a Type 1 Interferon from initiating an innate immune response. Therefore, the female's urogenital mucosal immune system will not be stimulated to produce an inflammatory response. (http://www.biolreprod.org/content/49/1/162.full.pdf)
The Hu-FEDS hypothesis suggests Glycodelin A is able to suppress lymphocytes and NK cells in vitro so, possibly this can explain why the mucosal system does not react. However, its dose dependent fertility suppressive effect on sperms also raises some questions that I will further searching to understand. (http://molehr.oxfordjournals.org/content/3/2/81.full.pdf) By the way, for anyone who understands the Hu-FEDS hypothesis, is it a significant pinnacle in reproductive physiology that has been accepted as a theory?I'm asking because I have had trouble finding recent publications i.e. 2005 onwards.