Insane... I know

So I'm sure this is stupid to ask but I'm going to ask it anyway. And no it's not because I'm not ready and its not because I'm stupid.. But how do people/couples not get pregnant from having sex?! I know there's obviously forms of contraceptives and birth control, but what about the case (that I am more speaking about) of not using ANYTHING? like my parents for example, they never use anything and only had three kids.