That's not really true either.
In India, people are generally constrained to marry within their caste (varna = color). Most historians believe caste rose out of conquerors of a different "race" trying to keep their race pure.
In most of these United States, interracial marriages were illegal and invalid for a very long time indeed.
Many other societies have had restrictions on intermarriage between groups that have some semblance to laws against interracial marriage.
I do note that the idea of "race" in the modern sense we take for granted is a unique idea. Only developed in the last few hundred years.
In other words, the comparison of same-sex "marriage" to interracial marriage is bogus, a trick that is only based on the past local legal constraints, same as applying the bogus battle-cry of "civil rights" in just about any issue that doesn't have a real, normal justification.
You can try http://www.pmail.com
It’s not an open source, but it’s free (donations welcome). David Harris is in NZ and has done an amazingly good job. There are also encryption / decryption products, extensions and plugins, mostly based on PGP.