Nothing provides a child with a mother or a father for life. There is no provision in American law for contractual imposition.
Nevertheless, the closest we come to the tortured obligation of a third party is the third party beneficiary contract. That it's not applicable, or even useful, only illustrates that there is nothing in our laws that remotely applies.
I personally feel that imposing same sex parents on a child is a form of child abuse. There is no legal argument against same sex parenting. Raising children into twisted, dysfunctional adults is not against the law.
Oh but there absolutely is a legal argument. Marriage is a contract between two adults because of children. You'd have to prove that isn't historically true and contemporarily important.
No, Sil. You are wrong. There is no argument that would legally or morally justify discrimination.
Marriage is a "personal relationship" between two adults. Their civil contract consists of mutual obligations of respect, fidelity, and support. Children are not parties to the marriage contract nor third party beneficiaries of the marriage contract. Skylar has pointed out to you numerous times that the things you imagine are not the law. Although the marriage partners might have children, their contractual marital relationship does not exist "because of children".
If a marriage partner breaches the civil contract, i.e., violates the obligations of respect, fidelity, and support, then the other party to the contract has legal grounds to terminate the contract (dissolve the marital relationship). The aggrieved party may seek a divorce (dissolution of the marriage) based on "fault" such as mental cruelty (lack of respect), infidelity, or non-support. The law has evolved to provide the parties with "no fault" grounds for divorce, e.g., irreconcilable differences. The fact that the law does not force people to stay married "because of children" is strong evidence that your proposition is again a product of your imagination. No one has the burden of disproving the fallacies that swirl around in your head. We recognize that your proposition is borne from your personal animus and does nothing to protect children, but rather harms them as noted by our courts.
As a matter of law, children are not "third party beneficiaries" of the marriage contract. The only legal relevance of the designation "third party beneficiary" is that the third party has the right to enforce the contract. Those are the kind of contracts wherein the contract is made for the express purpose of benefitting the third person. However, a child does not have standing to petition a court of law and enforce his married parents' obligations to respect each other, to remain faithful to each other, and to support each other. A child cannot force his parents to remain married. That does not mean that a child does not benefit from his parents' marriage when the married parents honor their contractual obligations to each other; he does. But benefiting from a parents' good marriage is not he same thing as being a "third party beneficiary" of the marriage contract itself. If the marriage contract is dissolved, a court will take into consideration the best interests of the child when determining custody and support issues in an action for divorce. But an incidental beneficiary of a contract does not have standing to enforce a contract.
Many years ago, I read a child custody case where a court denied a mother custody because she was a lesbian who was living with her partner. This deprivation was based on the father's alienation of the children's affection from their mother because she was a lesbian (known as "poisoning the well") and the court's fear that the children would suffer from "the slings and arrows of a disapproving society." Much later, however, the court reversed itself finding those dreaded "slings and arrows" never materialized. Furthermore "poisoning the well" is a factor that weighs against the party who is engaged in the alienation.
Sil, your entire argument has nothing to do with facts or the law nor even the best interests of the children. It has everything to do with your personal disapproval of same-sex marriage partners. You use poison to hurt rather than help children to thrive. You're the one who is harming them with your slings and arrows, and the rest of us say to you: Mind your own business, you mean, mean, mean person. Tend to your own life and your own children.