I think all marriages should be banned. The reason is that when a couple (either hetero or homo) gets married, immediately the individuals of the couple get "rights". These rights, at least in Europe, are the rights to get pensions in some circumstances. The problem is these pensions are funded by taxes paid by individuals who have nothing to do with that marriage. This is an externality. Since it makes no sense that I have to fund payments to a couple when, for example, this couple divorces, I do not want that official marriages exist. Of course, private contracts are OK.