There is an exception for necessities (like food and shelter). A child can’t void those contracts, precisely so they have the ability to enter into them. If they could be voided, minors might not be able to obtain these essential items.
As to the OP, consent can be by word or deed. If a person never said “I do”, but attended the wedding, dressed appropriately, and otherwise participated in the ceremony, an argument could be made that they did in fact assent.
Ultimately, though, I believe the legal confirmation happens with the signing of the marriage certificate, and so it’s really just a religious standard that has to be met to be considered married after the ceremony. If the priest accepts a thumbs up in lieu of a more formal assent, who is anybody else to judge?
This sort of situations has been studied by J.L. Austin in How to Do Things with Words, in particular in the context of performative utterances, where saying something is actually doing something. These utterances are not descriptions and consequently neither true nor false, but they can be infelicitous if the wording is not exact, or the right conditions aren’t met.
As Austin observes, the acts purported to be performed by performative utterances may be socially contested. For instance, “I divorce you”, said three times by a man to his wife, may be accepted to constitute a divorce by some, but not by others.
Every performative utterance has its own procedure and risks of failure that Austin calls ‘infelicities’.: 14 He sees a sharp distinction between the individual text and the ‘total speech act situation’ surrounding it. According to Austin, in order to successfully perform an illocutionary act, certain conditions have to be met (e.g. a person who pronounces a marriage must be authorized to do so).: 8 Besides the context, the performative utterance itself is unambiguous as well. The words of an illocutionary act have to be expressed in earnest; if not, Austin discards them as a parasitic use of language.
I think number 3 here is where hypothetical meets reality, as many arranged marriages throughout the world wouldn’t really meet our traditional ideas of enthusiastic consent. If your parents, religious leaders, community elders, and everyone else you know is basically telling you who to marry, are you really consenting? Or is that social coercion?
It’s certainly not great that a 15 year old, even one who begrudgingly says “I do” at some point, can basically be told to marry someone they otherwise couldn’t legally consent to having sex with.
The standard Church of England wedding service is explicitly designed for the priest to be a sure as possible that both parties are consenting.
A check to see if anyone objects:
Therefore if any man can shew any just cause, why they may not lawfully be joined together, let him now speak, or else hereafter for ever hold his peace.
I REQUIRE and charge you both, as ye will answer at the dreadful day of judgement, when the secrets of all hearts shall be disclosed, that if either of you know any impediment, why ye may not be lawfully joined together in Matrimony, ye do now confess it.
N. WILT thou have this woman to thy wedded wife, to live together, etc.
N. WILT thou have this man to thy wedded husband, to live together, etc.
FORASMUCH as N. and N. have consented together in holy wedlock, and have witnessed the same before God and this company,
And, of course, the Banns of all that are to be married together must be published in the Church on three several Sundays, before the wedding to give anyone the opportunity to put a spoke in the plan.
That can vary considerably by jurisdiction. There was no requirement for a marriage certificate at common law (see @bob_2 's excellent post about banns). How much legal weight there is to a marriage certificate depends on the law of the particular jurisdiction.
In my jurisdiction, the certificate is evidence of a wedding that has already been performed (“I now declare that you are husband and wife” / “spouse and spouse” etc.) Even if the officiant never sends in the certificate to the relevant central authority, the couple is still married. The officiant may have committed a civil offence by not sending it in, but it doesn’t affect the validity of the marriage.
In Japan, any ceremony has no legal meaning and the only legal step in submitting the form to the relevant authority. Both my wife and I were there with the completed form, which requires two witnesses, and the clerk didn’t ask if we both consented to the marriage.
This is a translation into English of the marriage registration form, and it does not have any check marks for consent.
The requirements for marriages in Japan require family registries for citizens and certain other requirement for aliens, but fails to mention anime characters, so I presume that still is not recognized.
Of course, the assumption is that the presence of both spouses’ seals as well as those of the witnesses implies consent, but this system has flaws.
There are cases both of marriages and divorces (uncontested divorces only need an application to be submitted) in which the seal of the individual was fraudulently used without knowledge of the person.
While fortunately the number of cases is small, ISTM that submitting paperwork is much easier to hide fraud than a ceremony.
By our modern understanding of consent, yet. But I think that historically, coerced consent was accepted as consent. And consent (coerced or otherwise) was a requirement for a valid marriage. And there have been cases of people running away or killing themselves to avoid being coerced into granting consent.