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The picture shows a brother and sister who became known as a "married couple" to their friends.
I'd like to use a quote, in my reply. It goes "There is nothing sacrosanct about culture. Culture is constantly evolving in any living society, responding to both internal and external stimuli, and there is much in every culture that societies quite naturally outgrow and reject. We are thus not obliged to defend, in the name of culture, practices that treat an adult as incapable of making meaningful decisions, and instead empower the parents of an adult man or woman to determine their off-spring's partner in marriage.
One setback with culture is that it subsumes all members of a society under a framework they may prefer to disavow/renounce. If dissenters within each culture are free to opt out and assert their individual rights - for example a person's right not to marry under customary law - an adult of sound mind can legitimately opt out of cultural traditions." (End of quote.)
Basically, the quote is pointing out that the law against incest tells everyone, even intelligent, moral, adult citizens, that they can't marry certain people who they love. The law therefore is treating them as though they're incapable of making a decision for themselves as to who they should or should not marry. It violates our freedom to marry the person we love.
It puts us all in the same restriction, no matter how deep our love is, and no matter what our individual preferences are. It's autocratic. Just as we have the right to choose to follow our cultural traditions, we should also have the right to choose not to.
Source: http://www.newvision.co.ug/D/8/459/520634
Note: The quoted discussion was originally used in support of marriage within the same clan, but it equally applies to marriage within the same family.)