People have sexual intercourse because they enjoy the sexual stimulation and the physical and emotional feelings that come from contact between the penis and the vagina or anus.
Sexual intercourse happens when a penis is inserted in either the vagina or the anus (the bum). When sexual intercourse happens without the penis being covered with a condom, there is an exchange of body fluids. This is one way that STIs can be passed on from one person to another.
Vaginal intercourse can also result in pregnancy. Every time a male ejaculates (or cums) the penis releases about 500 million sperm. It only takes one sperm to fertilize the single egg that is released each month into a woman's Fallopian tubes (which are connected to the uterus, which connects to the vagina).
It's important to know that, sperm may be present in pre-ejaculate, if a male has recently ejaculated in the last few hours. This is because leftover sperm may still be present in the urethra before ejaculation. So the withdrawal method (pulling out) before ejaculation is very risky and can result in a pregnancy or the spreading of STIs.
Anal intercourse does not usually result in pregnancy but it can cause serious infections and it is one of the ways STIs are passed from one person to another if a condom is not used. People who have anal intercourse should always use a water based lubricant and a condom when having anal sex.