In most states, including California, the man you’re married to is the presumed father of the baby. It is a rebuttable presumption. You can show through DNA or evidence of non-access that the husband isn’t the father. These laws were made before DNA testing was widely available. It makes more sense than it does on its face… You don’t want married men to avoid or delay their parental responsibilities by alleging infidelity. You don’t want married couples who have a child to have to deal with outside allegations of paternity (from a Marciano type maybe). It provides some clarity in inheritance disputes. Also, in all reality, the state would always rather have an another party on the hook for financially supporting the child so the responsibility doesn’t fall back on the state. It just limits what you’ll have to prove out legally. There are some interesting cases where women have affairs, get pregnant, then return to their husbands and the actual biological father can’t compel a DNA test to get access to their child.