semen (containing sperm) tends to quickly liquefy after ejaculation. it can be thick and mucous-like at first but it tends to thin out rapidly. it is also mixing with your vaginal secretions so gravity is really your answer. the vagina is angled so that it drains. it drains menstrual blood monthly. deposits of semen will drain as well when you are more upright or during your sleep. hence the dreaded "wet spot".
why does my boyfriends sperm come back out after we have unprotected sex?