Last year the Constitutional Court of Colombia legalised same-sex marriages. Now three men in Medellin have gained legal recognition as a ‘polyamorous family’ unit with inheritance rights. The trio will hold a marriage ceremony and then go on honeymoon. Now that many people have decided that sexual complementarity is irrelevant to marriage, why should we allow only couples to marry? Why not three or more? While the union of the three men in Columbia is not marriage as such legally speaking, it is legally recognised and this could easily be a step on the road to polyamorous marriages in some countries. Many fail to see how demands such as these follow logically from support for same-sex marriages. Marriage, as understood everywhere in the world until very recently, is the union of a man and a woman. It is between two persons precisely because there are two sexes, each one complementary to the other. Both are necessary and sufficient for marriage. Once the essential component of sexual complementary is removed from the definition of marriage there is no logical reason why it should be limited to only two individuals. The State has no right to privilege or impose one form of family structure or sexuality over another, claim polyamory advocates. (As evidence of the still very small, but nonetheless growing demand for official recognition of polyamorous relationships see here and here. Note that these articles have appeared in mainstream, not fringe publications). The Irish constitution, post-2015, specifies that marriage is between two people. For pragmatic reasons, those who drafted the new constitutional definition had to make it as acceptable as possible for the general public and so they excluded multi-partners unions. However, most if not all of the arguments used to support same-sex marriage can also be used to support polyamorous marriages, or at least legally recognised polyamorous unions, for same ‘love is love’, ‘how is this going to harm you?’, ‘equality for all’. But there is a more compelling reason than this: children. Same-sex relationships are by definition infertile while polyamorous relationships including at least one man and one woman are fertile. Isn’t it in the interest of children born in polyamorous arrangements to have their parents in a recognised legal relationship? Or you can imagine a ‘throuple’ – yes, that’s the new word for three-partner relationships – where one woman and one man provide the egg and the sperm respectively, and a third woman carries the pregnancy. Why shouldn’t they all three be considered legal parents of the child and be united in one marriage? Or you can have three women (see here) who have a child via sperm donation all wanting to be legally recognised as that child’s parents. Once we decide that the natural tie to one man and one woman (the mother and the father) is no longer important, why not accede to their demand? All these novel arrangements suit adults but the question is how they are beneficial to children. Isn’t it better for them to have their own mothers and fathers dedicated exclusively to them rather than to other spouses and offspring? What is missing in polyamory is exclusivity. The husband doesn’t commit totally to the wife to the exclusion of all others, and vice versa. If marriage has to be, among other things, a sexual relationship, then it can only be consummated by no more and no less than a man and a woman. The conjugal act requires the two complementary sexes. No other act can in the same way unite three or more, no other act generates new life and has therefore the same relevance for society. Being ‘one flesh’ with someone entails commitment through time (hence permanence) and at each time (hence exclusivity). Once sexual complementarity is removed, and we lose sight of the importance of the natural ties, there are no logical reasons why other essential elements such as exclusivity and monogamy should be kept. (In my next blog I will discuss the recent ruling by the Irish Supreme Court on polygamous marriage, especially the argument that permitting such marriage would violate the principle of equality).