True there, but you forget that these facts can also have biologically reasoning behind it.

Yes, the value of sex can be made 'neutral' and determined by the cultural context it resides in, but it doesn't sway the biological factors behind it.

As an example, when a woman gets pregnant, her body goes through massive changes. Her skin darkens (which is why most men prefer glowing, lighter skin not in the racial sense), her nipples, genitals and lips darken (whereas they would have a more youthful coloured tint), her areola (the ring surrounding her nipple) grows larger, her breasts swell and droop slightly (which is why men prefer perky breasts as a sign of youth) - and many other physical signs which display that she has gone through a more mature stage.
A young woman will have a better glow, lighter coloured nipples, genitals and skin, redder lips, visibly flushed face (the blush), etc.
This is why women devise ways to look like she's at the peak of her youth, untouched by man and showing signs of her un-impregnated status even after she has aged or borne children, and men find such signs attractive and even find ways to encourage the exaggeration of such signals.

In many ways, our cultural values we place on these things such as virginity, less partners on women (and more stigma at more partners) and more for men (and less stigma at more partners) are based on the biological truths of our species, which we further embellish, exaggerate or tweak but the base stays the same.