I was gonna go with “gravity.”
Gravity and fluids probably explain the flushing of the bulk of the semen in the vagina and uterus, but there are hundreds of dead sperm and bits of dead sperm in the fallopian tubes and even inside the abdomen, if they live to swim so far.
I believe the female immune response in this instance is mostly a premptive one. Instead of fighting off sperm in the vagina, they frequently emply strategies to prevent it from entering in the first place, e.g. not returning phone calls, restraining orders, gossiping, panty hose, sweat pants, door locking, and Twilight movies. That’s how it seems to work for me anyway.
Apologies if somebody got here already but look up corkscrew (alternative?) vaginas in ducks. Brilliant!
FWIW.
Ready your lance, good sir, and CHARGE!