Why? --Evolution
jumb said:
But it's a big part of humanity, it's one of our core instincts. We only survive because of our desire to reproduce.
Jumb's got it. In fact, it's all about how much energy we've gotta put into the other person. Even if you ignore the traditional roles men and women have, and just look at childbirth/rearing (Which is what sex is really for... evolution doesn't care for our personal pleasure): women need to spend 9 months bringing up a child, and then typically years and years more after that. For a guy, potentially he just has to have sex, and that's it. However, humans don't tend to do that: we pair up, and so childrearing is a combined effort.
As a result of pairing up, we invest time and energy into the other person. The women, evolutionarily, tend to look after the children (guys don't have breasts), and the men do the leftover jobs (ie hunting).
Wit that in mind, why is virginity 'attractive'? Ignoring the fact that generally virgins are young and healthy (ie Komaticom), it means they don't have any other 'investments': no children you'll have to feed, no other men to compete with. They can rear your children, and carry on your genes. From there, as nick1048 said first, there're all sorts of cultural/religious additions to this, but it's easy to see how it's a basic instinct (as per Jumb), and how that's been expanded out into our culture..