The vagina is basically a closed-ended, muscular tube. Since it's essentially closed, anything that goes in must eventually come out. So just like normal vaginal discharges, anything left over (ejaculate, lubricant, etc.) after sex eventually flows out due to gravity. Some will flow out right away (as you are noticing) and the rest leaks out over the next day or so. Please note this does not mean that the sperm all leak out (though some do). Sperm are really tiny and they swim, so many of them have already moved on before the rest of the ejaculate flows back out. So even though gravity and the natural design of the body causes the fluid to eventually flow back out, keep in mind that if you and your partner are having unprotected intercourse you still have pregnancy and STI transmission risks.