Not an exception. Minors cannot get married if they're under 18 in any state unless they get their parent's (or a court's) written approval. A minor cannot enter into a marriage contract by themselves.

I’m no expert, but my Business Law professors were absolutely adamant about the fact that a minor can never execute a legal agreement and there are no exceptions.

No, you're absolutely right. There are no exceptions. In the US there are roughly six elements necessary for a contract to be legally binding, and this one is the issue of "legal capacity": does the party entering into the contract have the capacity to understand and be bound to such an agreement? For minors, the broad answer in law is always no. They can get emancipated (a judge rules you an adult for legal purposes), they can have a parent or legal guardian act in their behalf, they can postpone until they are of age, etc. But they do not, as a rule, have the legal capacity to enter into a binding contract.

That's what this is. Minors appear to enter into contracts all the time. Every time a minor clicks on a EULA they're purporting to enter into a contract. But that does not make it valid. Just because a signature is on a line that is not enough to make a contract binding and enforceable.

The defendants are pulling this because in the US we've entered into a new wtf era in the courts and they're simply hoping to get a judge to allow it to stick, but this whole "an eight year old successfully waived his rights" argument has no basis in law.

To put it another way I'm going to pivot and use a different example that more people are familiar with, and one that I care a lot about: underage marriage. Apologies in advance for the shoehorn but it illustrates the point.

This exact issue of being unable to legally enter into agreement, or provide legal consent, is why underage marriage is so problematic: it's a legal trap for the minor, and essentially a form of legal slavery. Children can get married, but children have no legal power to initiate a legal proceeding (like getting a restraining order or filing for divorce) nor direct their own health care, even reproductive health, unless they act through a parent or legal guardian. If they are mistreated they can't even be admitted to a shelter, since shelters have agreements and children cannot enter into contracts, period end of.

Legally, just because a child has married, they do NOT suddenly gain legal emancipation. This is another big misunderstanding. Emancipation happens solely by court order, not by marriage. Even the marriage itself required parental consent to be legally binding, depending on the exact age and location. The married minor is still legally a child, unable to execute or be bound to contracts. The exceptions to this are informal, such as when despite a child's minority, others allow a child to go through the motions and honor an invalid contract anyway, OR formal, when the child has a legal guardian that is willing to enter into contract and genuinely act on the child's own behalf.

And the worst of it is that in most states and under federal law, sex with a child that would otherwise be considered rape – in some cases, felony rape – becomes legal within marriage. In those situations, the marriage license becomes a “get out of jail free” card for a child rapist. That's how few legal rights children have in reality.

There are organizations working hard to turn this around, but it's an uphill battle. Here's more information for anyone who wants the actual legal picture: https://www.unchainedatlast.org/child-marriage-in-the-u-s/