Some commentators, such as McGill University law professor, Margaret Somerville, argue that same-sex marriage threatens the very nature of marriage and that it poses threats to children.
In particular, Somerville argues that same-sex marriage is alien to the deeply symbolic meaning of marriage as open to having children. When the Court of Appeal for Ontario opened the way to same-sex marriage in Halpern v. Canada, it noted that – in fact – some same-sex couples do have children, but the fact that gay men and lesbians do have children does not mean that their relationships are fertile in the sense that is meant by critics of same-sex marriage. Their view of marriage is, however, only one among possible meanings of marriage, and there’s no good secular reason to accept the view that straights only need apply when it comes to state-recognized marriages.
Now, lots of critics have argued that gay and lesbian parents are ‘bad’ for children, and critics of same-sex marriage make this case in different ways. Some think these kinds of parents inflict real harm on children; others thing the harm is minimal but should be avoided anyway. In fact, however, the courts have typically not taken sexual orientation into account when deciding matters of custody, and – from an ethical point of view – it is hard to make the case that gay and lesbian parents are harmful to children in any routine or any rare-but-catastrophic way. Leading professional organizations have also rejected the view that this kind of parenting leads to harm to children.
I make these arguments in detail in the article “Same-Sex Marriage: Not a Threat to Marriage or Children,” Journal of Social Philosophy 2011 (42): 288-304.