Marriage seems universal, occurring at all times and in all cultures. The implications of marriage differ vastly across countries, yet it seems that all countries recognize marriages (at least if among consenting, hetero-sexual adults) performed in other countries. Is this so? Are there any countries, any legislations, where the concept of marriage simply doesn't exist as a legal construct?
Not sure if this matches exactly what you seek, but there's one problem with finding such a country (or help :) - it must NOT have adopted the "UN's Universal Declaration of Human Rights", which states:
(1) Men and women of full age, without any limitation due to race, nationality or religion, have the right to marry and to found a family. They are entitled to equal rights as to marriage, during marriage and at its dissolution.
I'm not aware of any such country that both have not signed, AND doesn't have a marriage law.
If you don't confine the question to current, UN-recognized countries: