Can semen/sperm run down from inner thighs into vagina and cause pregnancy?

Yes, but...: In real life, that usually does not happen. There generally is not enough semen to run down a thigh, over the vulva, an into the vagina. Ejaculation usually has to happen into the vagina in order to create a new baby. That being said, life can surprise you sometimes. So, couples having sex should always be ready for that rare surprise baby.