Is it normal to have semen drip out of your penis after you ejaculate?
Consider what happens when you run the sprinklers to water the lawn. Even after you turn off the water at the faucet, water continues to dribble out of the other end of the hose. This is because the pressure to move the water is at the end by the house. You can cut the pressure, but all the water still in the hose will move out, though at a much slower rate because there is no shut off valve at the end by the sprinkler.
Your urinary tract is much the same. The pressure for ejaculation comes from the prostate gland, but the pressure doesn't completely empty the urinary tract. There are no muscles in the penis to close off the end of the urinary tract, so what is in the tract will dribble out. In addition, the Cowper glands, which are producing clear liquid, called pre-ejaculate (or pre-cum in slang), continue to produce pre-ejaculate even after orgasm. This pushes any leftovers out the end of the penis as well. All of this takes place at a much slower pace than ejaculation.