Actually, it's only since the onset of patriarchy, about 6-9 thousand years ago. From time immemorial, modern anthropology agrees that cooperative breeding and allopartenting (where many others than the parents) help to raise children and this is how humans survived and thrived. Besides, before the coercive social and sexual control of women that only arose with patriarchy a few thousand years ago, how was anyone to actually know who a father even was? Besides, it didn't matter in tribes where the primary survival strategy was providing for and taking care of everyone.
The idea that men live in fear of raising another man's child is also a patriarchal construct. Partible paternity is widespread in South America and parts of Africa, where two (and sometimes more) men are considered the father of a child after the mother mates with both of them. Two fathers seems to be quite a beneficial arrangement for the children, providing greater reproductive fitness than having only one.
“A female who mates with several different males will have more genetically diverse offspring, boosting the chances that at least some of them will thrive.” Source This goes for human females as well as other non-human animals and it is confirmed in the anatomy of both human men and women.
The coronal ridge of the human penis is specifically shaped to displace semen left there by another man or men. In addition, animals that engage in mate competition prior to copulation (like gorillas) tend to have small testes and penises. Animals that instead engage in sperm competition are more well endowed relative to body size (like chimpanzees and humans) because they need to have a large supply of semen on hand to inseminate multiple partners."
https://medium.com/sensual-enchantment/is-monogamy-natural-for-humans-ff1889bcb26f
There are also rather a large number of societies that have a marriage-like structure but have no real expectation of sexual fidelity, at least not all of the time.
The fact that men become aroused and "inspired" by watching other men mate isn't about patriarchal competition. Again, that's a very, very recent human concept. Our ancient ancestors survived and thrived due to their capacity for cooperation. Yes, we are natural multi-maters, just like our chimp and bonobo cousins, and sperm competition is where the action is at (not fighting for mates like gorillas or elk). So yes, being able to shoot more genetic material further gives you an evolutionary leg-up so to speak.
I think there are a lot of things that we agree on here, but patriarchy has only been around for about 3% of human history so portraying it's dynamics as timeless and "throughout history" as you do in some places doesn't really paint an accurate picture.
