The people of those towns [in Tibet] have a strange custom in regard to marriage which I will now relate.
No man of that country would on any consideration take to wife a girl who was a maid; for they say a wife is nothing worth unless she has been used to consort with men. And their custom is this, that when travellers come that way, the old women of the place get ready, and take their unmarried daughters or other girls related to them, and go to the strangers who are passing, and make over the young women to whomsoever will accept them; and the travellers take them accordingly and do their pleasure; after which the girls are restored to the old women who brought them, for they are not allowed to follow the strangers away from their home. In this manner people travelling that way, when they reach a village or hamlet or other inhabited place, shall find perhaps 20 or 30 girls at their disposal. And if the travellers lodge with those people they shall have as many young women as they could wish coming to court them!
You must know too that the traveller is expected to give the girl who has been with him a ring or some other trifle, something in fact that she can show as a lover's token when she comes to be married. And it is for this in truth and for this alone that they follow that custom; for every girl is expected to obtain at least 20 such tokens in the way I have described before she can be married. And those who have most tokens, and so can show they have been most run after, are in the highest esteem, and most sought in marriage, because they say the charms of such an one are greatest.
But after marriage these people hold their wives very dear, and would consider it a great villainy for a man to meddle with another's wife; and thus though the wives have before marriage acted as you have heard, they are kept with great care from light conduct afterwards.
Source:
Polo, Marco: The Travels of Marco Polo. Book 2, Chapter 45: Concerning the Province of Tebet
Further Reading:
"Honey, I'm going to Tibet on business have you got any spare subway tokens?"