Argument: Gay exclusion is just to protect procreative marriage
Margarette Somerville. "The case against gay marriage." McGill Center for Medicine, Ethics and Law. April 29, 2003: "People advocating same-sex marriage argue that we should accept that the primary purpose of marriage is to give social and public recognition to an intimate relationship between two people, and, therefore, to exclude same-sex couples is discrimination. They are correct if the primary purpose of marriage is to protect an intimate pair-bond. But they are not correct if its primary purpose is to protect the inherently procreative relationship of opposite-sex pair-bonding or to protect an intimate relationship for the purposes of its procreative potential. When marriage is limited to opposite-sex couples, there is no need to choose between these purposes, because they are compatible with each other and promote the same goal. The same is not true if marriage is extended to include same-sex couples. That would necessarily eliminate marriage’s role in symbolizing and protecting the procreative relationship."
The reason for excluding same-sex couples from marriage matters: If the reason for denying same-sex marriage is that we have no respect for homosexuals and their relationships, or want to give the message that homosexuality is wrong, then, the exclusion of same-sex couples from marriage is not ethically acceptable from the perspective of respect for homosexuals and their relationships. It is also discrimination. On the other hand, if the reason is to keep the very nature, essence and substance of marriage intact, and that essence is to protect the inherently procreative relationship, then excluding same-sex couples from marriage is ethically acceptable from the perspective of respect for them and their relationships."
Margarette Somerville. "The case against gay marriage." McGill Center for Medicine, Ethics and Law. April 29, 2003: "The primary intent in restricting marriage to opposite sex couples is to maintain marriage as the institution that fulfils society’s need to protect the inherently procreative relationship and its functions for society, and is not to exclude homosexual relationships because they are homosexual. The discrimination involved in the exclusion is a secondary effect which is not desired but unavoidable, and it is justified or excused by the primary purpose which otherwise cannot be realized. [...] A useful comparison can be made with the discrimination involved in affirmative action. That shows that sometimes discrimination and the harm it involves, can be justified when it is to achieve a greater good that cannot otherwise be achieved."