A thought on this topic:
I sometimes imagine primitive man's early days with a groups of people devoid of both religion and civil order. For any number of reasons mankind adopted a form of what we call marriage. There was possibly little or no ceremony, maybe an exchange of valuable items as a show of affection. Some kind of simple social arrangement. What ever it was, it set them apart from the rest, and likely had some benefit for the raising and populating their areas of habitation. Families and larger Tribes were formed. Primitive forms of society accepted and enhanced this union for reasons we can try to guess at (reproducing families comes to mind), but it became ingrained in many different cultures.
Civil orders and Religions adopted and encouraged this convenient arrangement and blessed and enhanced its role with ceremony and official public notification. This is important since both Religion and Civil/Social orders significantly strengthened the only taboos for relations outside of marriage. Along the way, because of the way societies function, additional benefits were added to this recognized public unity. The ability to pass on inheritance freely without dispute, benefits of association, recognition, income, property rights, and so forth are passed on through the family which only comes after marriage.
At no point along the way until now has there been a real social need to have same sex unions. We must ask ourselves why? If we can find the fundamental reasons for why things are the way they are, we can discover wither changes to this natural progression will be harmful or beneficial. I doubt that the problems if any would be recognizable right away, but would have to be thought of in much longer terms than the here and now.
