The vagina is pretty much like a dead-end street
for the semen, though the sperm (which are much, much smaller) will swim up into the cervix and the uterus and the tubes. However, the gush of semen is mostly the bus that takes the sperm to the cervix, where it is left behind as they swim from there. That means that when the woman stands up, or sneezes, or pees, the vagina will push out the semen.