And this isn't true either, except for the past 6-9 thousand years when patriarchy arose, and not even universally. There are many current cultures where partible paternity takes place or where paternity is not considered to be that big a deal.
Marriage, Monogamy and the Nuclear Family Are Not Human Universals
In many cultures, there is no expectation of fidelity or only two parents
“Despite the belief that monogamous male-female bonding is how mothers and children were supported and thrived, the anthropologist Sarah Blaffer Hrdy and others believe it was actually female cooperative breeding, or alloparenting — ‘sharing and caring derived from the pooled energy’ of a network of ‘grandparents, aunts, uncles, siblings, distantly related kin, and non-kin’ — that shaped our evolution.” (2)
In most of rural Africa children wander in and out of the homes of unrelated adults in their villages, where they are treated as family. This stems from a more community-oriented lifestyle in general, but also from a lack of concern for parentage because sexual exclusivity is not prioritized.In fact, in some cultures, it is considered downright stingy.
Meanwhile in Brazil, In a traditional Canela marriage ceremony, the bride and groom lie down on a mat, arms under each other’s heads, legs entwined. The brother of each partner’s mother then comes forward. He admonishes the bride and her new husband to stay together until the last child is grown, specifically reminding them not to be jealous of each other’s lovers.