There is a degree of certainty for married parents. Children of the marriage are, for example, legally presumed to be the biological children of the parents (at least with heterosexual couples). In the absence of a premarital agreement or other special arrangement, when a California marital relationship ends, assets divide according to state community property standards and child custody arrangements move forward per the children's best interests.
Pew Research shows that one of out every four parents living with a child in 2018 is unmarried. If unmarried parents separate, things are a bit more complicated. The complication extends not only to the division of property that the couple may have shared in their time together - which can itself be very difficult to divide depending on how long the relationship was and how commingled the assets are - but also to custody arrangements.
A higher percentage of parents are choosing to remain unmarried but are raising their children together, which is a personal choice that has pros and cons for the families involved. One definite point of contention in an uncommitted parental relationship is if the parents split and there is a custody battle between them (or a dispute about child support).
When parents are unmarried, the male is not automatically legally presumed as the child's father. As such, it is difficult to determine paternity for purposes of paying child support or seeking custody or visitation. It is certainly possible for a man to accept parentage, and to sign his name to a child's birth certificate at the time of the birth or afterwards. If that step isn't taken, though, then paternity must first be determined before moving on to custody, visitation, and child support.
There are certain legal rights and obligations that come with parentage. A man who isn't recognized as a legal father may not have the responsibility to pay child support (he may need to pay back the state for benefits given to the mom during the pendency of a paternity proceeding), but he also does not have the right to seek visitation rights or custody.
If you, as an unmarried parent, find your relationship ending and you are concerned for your children's best interests, strongly consider seeking the advice of an experienced family law attorney who can guide you through the processes of establishing paternity, creating a custody and visitation arrangement and securing a child support order.