A suggestion? Have your girlfriend sign up at a website for tracking her period, or download an app to do so. That way you can keep an eye on her fertile days and know better than to mess around without being very safe in the 6-8 days prior to her ovulation and thus peak fertility. Women are only really fertile for a few days a month, and sperm can live up to 7 days or so in the woman's pelvic cavity; so start tracking her cycles to get an idea as to when to be more careful!

As to your question: believe it or not, the vaginal pH is too acidic for sperm, and therefore it is actually toxic to it. Cervical mucous, which is more heavily present during ovulation, protects the sperm from being killed by the vaginal secretions. Seminal fluid also protects sperm. But with the potentially tiny amount present on your fingers, unless you were actually pressed up against her cervix (unlikely, as that would be very uncomfortable for her) would be ridiculously unlikely to survive enough to get up unto the uterus and head for the Fallopian tubes.

And with no offense at all intended, ladies and gentleman, this is the result of the lack of comprehensive sex education in schools and/or by the parents.