Which side of the aisle do guests sit on at a same-sex union?
Traditionally, guests of the bride would sit on the left side of the aisle (when facing the front of the venue) and guests of the groom would be seated on the right. However, at weddings with a very uneven guest list where one side would have considerably more people, ushers might be instructed to seat guests evenly on both sides, regardless of who's "guest" they are. This would keep the seating balanced and not draw attention away from the main event: the wedding ceremony taking place.
There is no way to assign sides when there are two brides or two grooms—so same-sex couples are free to assign guests a side arbitrarily if they would like (Sarah's guests on the left and Jennifer's on the right), or they might choose not to assign sides at all, and instead have their guests mix evenly. If a couple does choose to seat the guests according to which member of the couple they know better, this should not be read as a sign that one member of the couple is now playing the role of bride and the other of groom—same-sex couples should simply be themselves, and not be forced into any roles they don't choose to assume. The beauty of this question to me is that, just like a heterosexual couple, same-sex couples should feel free to use tradition when it is meaningful and relevant to them, and to abandon or recreate it when it makes sense for their unique situation.