That question forms the core of Margaret Somerville's case against same-sex marriage. She's the director of the McGill Centre for Medicine, Ethics and Law in Montreal:

Same-sex marriage creates a clash between upholding the human rights of children with respect to their coming-into being and the family structure in which they will be reared, and the claims of homosexual adults who wish to marry a same-sex partner. It forces us, as a society, to choose whether to give priority to children’s rights or to homosexual adults’ claims. This problem does not arise with opposite-sex marriage, because children’s rights and adults claims with respect to marriage are consistent with each other.

Many people who oppose extending the definition of marriage to include same-sex couples do so on religious grounds or because of moral objections to homosexuality. In contrast, my arguments are secularly based and, to the extent that they involve morals and values, they are grounded in ethics, not religion.