RE: What about marriage makes sex sacred?
Historically, marriage was important in order to have clear lines of succession and property rights. That was back in the days before the real prevalence of the romantic view of marriage for love. Hell, back in those days, even the peasants didn't marry for love, but for mutual advantage. Love often developed later, but it wasn't considered as important.
As has been said, with no reliable methods of birth control, kids born out of marriage could only confuse and dilute the rules of inheritance and property ownership. Thus, formal legal marriage was considered necessary. Cultural preferences, exclusively patriarchal, demanded at least feminine virginity, while the boys could fuck whoever they could. Religious interpretations and influence over marriage came late in the process, and it's only been within the last 1,000 years or so (a mind-numbing time, but still comparatively short compared to the existence of our species on the planet) that the whole Biblical interpretation of marriage has come to be important.
It's not that it wasn't there (although if the Nicean council had decided differently, it wouldn't be), but it wasn't interpreted as being significant until the religion adapted to the social practices (not the other way around). Then, all of a sudden, it's significant to draw parallels with Adam and Eve (and let's forget all the multiple wives and concubines and sleeping with in-laws and things in the Old Testament as inconvenient to the argument), and people start being raised thinking marriage was ALWAYS like this, and ordained by God, when frankly? Not really true. Same thing with sex, that's later Church interpretations casting sin on what didn't used to be really sinful, per se.
People think virginity is important because the concept of "purity" is ingrained in us pretty early in a variety of ways, and virginity is the mark of sexual purity. But of course, there has to be an outlet, an acceptable end-point to the purity, or else the species dies (look at the religious sects like the Shakers who gave up sex altogether and whose communities died out when they couldn't get enough converts to replace those who died). And also, sex is a mighty powerful impulse in humanity as a whole. So you have to have an acceptable outlet, and we're also trained that marriage is that. That way, any kids that come from it are legal, inheritance is covered, and the social construct can continue.
Virginity and marriage and their connections are only important because we think they are. Whether we make conscious decisions in that respect or we just go along with what we're trained, by parents, religion, or culture, that's up to the individual.