"Why is God, the creator of the universe, so interested in what semi-evolved hairless apes do in their sex lives?"

Because our sex lives are where we decide what kinds of new humans get made, i.e. how evolution unfolds, i.e. what the human race becomes, i.e. what values (if any) colonize the universe.

God wants people to be faithful because paternity uncertainty erodes the male incentive to invest resources into children; cultures with absent fathers and slutty women are s*itholes because the men have no incentive to build.

God wants women to marry young (18-22) because years of playing the field erode a woman's degree of attachment and sow the seeds of infidelity & broken homes, which lead to damaged children.

Now, I want to disclaim that not everything that "God" wants must be granted.

It is easy to imagine - or even just notice - civilizations that have some really, really oppressive traditions. Female Genital Mutilation being the prime example.

But there it is - the fact that the bible is obsessed with our sex lives is not at all a coincidence.

The future of the entire universe *does* actually depend on what people do with their sex lives, because sex choices are the process by which we shape our future.