Marriage.
But how are families formed? Well they are formed by family units which are at least here in Mexico formed by a social group of two or more people, related by blood, marriage, or adoption, who usually live together. What never changes is that families are formed by marriage between the father and the mother, a marriage is a legally sanctioned relationship, usually involving economic cooperation as well as sexual activity and childbearing that people expect to be enduring although at this time many people are not taking marriage seriously or marriage to early and end divorcing which usually is bad but there are cases when one of the integrands of the marriage was abusing of the other or being cheating with another woman/man in any of this cases it make sense that they would divorce.
Types of families.
According to what I have found there are three types of families which are: couple families, lonely families, and other families. As the name suggests couple families are formed by a couple of any type gay or straight and their children this is of course the most popular type although many people oppose gay couples but they are wrong gay are as capable as the straight. Then there are the lonely families which again are pretty self-explanatory which is only one parent and his children many although not that much have this type of family. And the last one is the others which are rare cases like brother and sister without parents and things of that style.