OK, let me clarify, I meant that although saliva is said to create an acidic environment, when the semen shoots forth, the sperm go from the end of the penis up into the cervix. Unless you put a whole lot of saliva all the way into the far end of the vagina (the end away from its opening, which would take some doing), it won't kill all of the millions of sperm that come out in an ejaculation. The acidity of the saliva would be further down, and left by the wayside, so to speak.
If you are simply talking about having given some stimulation by mouth and then the guy finishing in the vagina, the saliva won't bother anything any more than a slightly acidic vaginal environment would, even if saliva was hostile to sperm.