dear Dr,Naseer 

sperm consider as antigen for man and female as you know in healthy men, sperms do not come in contact with blood. The blood-testis barrier formed by the Sertoli cells in the testes keeps the sperm away. if this barrier  is opened the anti-sperm antibody occur.Also, in women may  develops antisperm -antibody.  For your question,  I think the time is avery important here where the sperm may reach to target egges before the immune respose takeplace .

best regard   

I think sperm has hidden antigen make it undetectable by female immune system.

you can read my paper 

 I think it was not detected by female immune system because it dead, so the immune response was activated by living foreign body, in other way sperms was foreign body 

Because the sperms do not come in contact with blood in the vagina. Hence, no immune response against the sperms is caused in the woman.

https://www.thehindu.com/sci-tech/health/why-is-it-that-sperms-are-not-considered-foreign-by-the-immune-system-of-women/article7569255.ece

Also, kindly check:

Article Manifestations of immune tolerance in the human female repro...

Article Sperm success and immunity