He has an orgasm causing him to ejaculate, and sometimes, she has an orgasm, causing the sperm to be pulled further up into her body to raise the probability of becoming pregnant. That's sex, the simple biology. Being responsible about your sexuality is an important part of having sex. First, I caution girls to not have sex too early. There are several reasons.
Sex causes pregnancy.
If you have sex, you're taking a huge risk of altering the course of your life forever and limiting most of your choices. Throughout most of human history, people married and created families in their teens. That had to do with survival of the species. Make babies when you're young and strong and before you die in your thirties or forties. Today, most people aren't even finished with school until late teens or early twenties.