It's normal for semen to leak out after sex. You have to keep in mind that your vagina is not bottomless. The opening to your uterus is at the end of it and that opening (your cervical canal) is tiny, so only a small amount of semen gets in (look at diagrams of the female reproductive organs). A drop of semen can contain more than a million sperm. The rest leaks out, it has no other place to go. To keep more sperm in after sex, you can lay on your back for a few minutes and prop pillows under your hips and thighs.
Edited to add: The more ejaculate a man puts 'in there', the more leakage you get.