Men, in general, really like having their genes carried on. Men, in general, really hate wondering if a child is theirs or not.
Sex by its nature completely shatters any notion we have of being civilized, rational actors made of pure white spiritual light. Alas, it shows us as sweaty, ape-like animals needing, nay, wanting to exchange fluids to produce our offspring. Nothing about it is pure and orderly and it is completely at odds with common mental strategies handling our issues with mortality. Being made of divine spiritual energy is quite at odds with the actual reality of it all.
Oh yeah, that and it being a means of production: it makes workers. Which is to be controlled at all times. But that alone does not explain why this taboo is also common in other type of societies.