If a women would jump ship, it is good to discover before marriage. So do the guys a favour by courting each other's girlfriends..
This would help to sieve out the unfaithful ones making them the leftovers for foreigners in the country.
You assume that people, both men and women, do not change and remain faithful after marriage.
Marriage is an overrated thing, infused with man-made beliefs and values e.g. basic building block of society, holy matrimony, expression of true love, something that is intrinsically good etc.
If you remove all that additional layers of bullshit, marriage is nothing more than a business transaction between a man and a woman, and an alliance between two families. That is all.