If a male rubs his penis on the opening of a female's vagina, is there a possibility of pregnancy?

Chances of pregnancy: If there is any exposure of the vaginal area, whether inside or outside, to ejaculate (pre-cum or full ejaculation), then there is the possibility that pregnancy can occur. Sperm can be mobile within vaginal secretions to get to the uterus.

If sperm is: Transmitted into the vagina or at the introitus (tissues at opening of vagina) then pregnancy is possible.