This likely isn't far from the truth, "straightness" is very much associated with culture being oppressive/biased against queerness, and most modern theory on the matter concludes that in a vacuum pansexuality, to varying degrees, is the "norm", and that near-complete heterosexuality and homosexuality are the outliers. Similar thing with monogamy, humans are likely to exhibit non-monogamous behaviours if there's no cultural influence involved – in fact, humans are the only great apes and one of the very few (in the single digits) simians to be observed participating in monogamy. Pretty much every species that is similar to us is primarily or almost entirely non-monogamous.

Outside of cultural pressures for or against certain sexual behaviours, you generally see pansexual and polygamous tendencies in most of the population – although pansexuality isn't as common as non-monogamy, in both humans and other primates; but it is still common in apes in general – especially bonobos, who are extremely similar to humans behaviourally and in the most related genus to us, are extremely social even for primates, and who have demonstrated many archaic human behaviours & advancements, even making/utilizing hunting tools like spears and stone tools devoid of any human influence.

Anyways heterosexuality being the norm is likely mostly the product of cultural pressures rather than "nature". And if we were to somehow magically erase cultural pressures or even the construct of sexuality, most people would exhibit at least somewhat pansexual behaviours, even if they had a strong preference for one sex/gender/whatever the fuck you wanna call it.

*Fun fact, bonobos have been shown to be able to competently & rationally communicate in complex English (complex, as in around as well as a conversational/intelligent 3-year old) and can even understand & respond to human speech accurately; they can have a lexicon of hundreds of words, associate those words with concrete ideas/real objects with near-100% accuracy, and distinguish "fake" words from "real" words in English speech. They're the only non-human animals to have been proven to have the ability to actually understand and communicate in human language (both with humans and with other bonobos), and understand human spoken language. They're also in the stone age (an equivalent of it, at least) and can be observed in the wild making intentionally cutting-edge tools (unlike chimps), and also have separately been trained to manufacture Oldowan human archeological industry stone tools and have found ways to use them, but I'm more interested in linguistics than that. It's kind of sad that people sensationalize fraudulent cases of alleged human language comprehension in other non-humans, like Koko the gorilla, when you have the actual phenomenon legitimately right here... It's also sad that bonobos and chimpanzees are caught in the middle of human warfare in the Congo and are being hunted for meat, and are predicted to become extinct within a few decades because of humans... a majority of simians/anthroform primates are likely to go extinct soon, both because they're being eaten and because they're being sold off as exotic pets. We're literally hunting, slave trading, and cannibalizing our closest relatives to extinction, some of which are capable of early human-like thought & society/culture and spontaneous invention of technology only ever seen in archaic humans.

I say we do this: Bonobos in capitivity can already light fires, and they use that cook food. Bonobos and chimapnzees are also highly cultural & social animals, and tool production & cultural customs have been observed to spread within groups and between groups (even of different great ape species/families). We should teach a group or a few of bonobos this skill, watch it spread, and destroy humanity and store the survivors in vats or something. Then, in a few million years, bonobos will have evolved modern human intelligence and sophisticated futuristic technology, and then they'd bring us back to life and out of the vats into the future. And then boom, humans and future bonobos live together happily forever after. It'll be just like Stellaris (not the genocide part though). I'm only joking of course...