Well yes, if your girl friend is on sevista, then yes, it is a contraceptive. If you did not ejaculate, chances of pregnancy are very minimal.
Pecum is a clear fluid secreted when a male is aroused. In chemical composition it is close to semen but has no sperms in it. If a person has ejaculated in the last 24 hours, then some sperms remain viable in his urethra. These sperms may mix with the precum or preejaculate fluid secreted subsequently while this fluid passes through the urethra. So if a person has passed urine several times or if more than 24 hours have passed since the last ejaculation, there will be no alive or viable sperms in the urethra. However women are known to get pregnant if the precum entered their vagina and the man had masturbated in the last 24 hours or had normal sex. Hence though the quantity and quality of sperms is not good enough sometimes, it may be good enough rarely to cause pregnancy especially if an ejaculation happened in last few hours and the woman is close to her ovulation. However, since she is on sevista, chances are minimal. Take care!