I think on a purely physical level, once women have lost their virginity they are seen as not quite as good to have sex with, because the tightness of their vagina will have been compromised. Obviously the degree of this is infinitesimal and could only possibly be measured over prolonged time (assuming only normal intercourse is undergone).

In the case of a man's virginity, it is just the opposite. The penis loses nothing of its vigour and may well become a more effective instrument in time.

Therefore, popular conceptions about women with "t**ht p***ies" lead the crowd to favour virgins more highly.

This is only one aspect of a topic which also includes issues of a woman's virginity being part and parcel of the ownership of her husband (historically, most world cultures do not have a tradition of female dominance, thus the reverse would not work for men); quid pro quo, violation of that virginity could be seen as an attack on her husband more than to the woman herself.

At the end of it, I think the differing emphasis of male and female virginity lies in the objectification of women in varying, centuries old ways.