The argument is not ridiculous. It is reasonable. The flaws in it are finer, and need more detailed addressing.
The argument absolutely is ridiculous and unreasonable, and it is risible that you feel your provided justification amounts to "finer details," as if its crude nonconsequentialist underpinnings are somehow "nuanced" or "complex."
Note that even when adults are victims, fraud is a fundamentally consensual crime.
Firstly, there is an obvious distinction between legality and ethicality, notwithstanding the notable overlap between the two. Just because something is illegal does not necessarily mean it is unethical; conversely, many unethical acts (e.g., the capitalist exploitation of workers) are legal.
Second, the reason fraud is generally unethical is that it generates distress in the victim, or else curtails their level of happiness. Instances of fraud or trickery that result in neither, including those that somehow improve participants' wellbeing, are not unethical by any reasonable definition of the term.
The idea here is that virginity/chastity is an expensive long term asset, like a house or education.
This idea is traditionalist, prudish, sex-negative nonsense; actually, as I elaborate in this essay, it underpins the stigma against pedophilia as it was originally encoded into law in feudal England and subsequently adopted by the American Colonies:
England “codified its [first] statutory rape law in the Statute of Westminster of 1275,” which stated that “The King prohibiteth that none do ravish . . . any Maiden within age.” “Colonial American statutory rape law basically imported this language”; “[t]he idea behind such laws” in early American history . . . “was less about the ability or lack thereof to consent to such activity on the part of the female, and more about protecting white females and their premarital chastity--a commodity--as property.”
During this time, there were no illusions about "trauma" resulting from adult-child sexual interactions. Instead, the stigma functioned merely to sustain arbitrary norms and values relating to the institution of marriage (which, of course, is oppressive).
The idea that violating these norms is universally unethical, regardless of its impact on people's wellbeing, is nothing more than primitive nonconsequentialist "ethics." Given the commonsensical notion that, ethically speaking, consequences (particularly as they relate to people's happiness and suffering) are paramount, nonconsequentialism is not a valid or healthy ethical philosophical orientation. On the contrary, since it allows for and can potentially justify any sort of evil or abuse, it is decidedly unethical and socially harmful.
A basic formulation of my code of ethics, which is a variant of utilitarianism, is as follows: When determining the ethicality of decisions, what is pertinent are a person's intentions, the net effect of his decisions on societal happiness, and the difference between his intended and the actual effects. Please explain 1) why you subscribe to nonconsequentialism despite its potential dangers and indifference to human happiness/suffering, and 2) specifically why you take issue with my formulation.
In case of sex, it is assumed that the child suffers a great loss, and the adult benefits.
This assumption is based on oppressive norms that are indifferent to the child's wellbeing, rather than a genuine concern for any actual suffering incurred; the usage of "suffers" here is therefore a misleading misnomer.
The fact that the child consents merely proves "fraud"
This is a faulty analogy fallacy. Whereas victims of criminal fraud are not apprised by the perpetrators of the full truth of the transactions between them, adult-child sexual interactions do not necessarily involve trickery or some kind of harmful obfuscation of the truth, especially not when the latter consents.
no reasonable and adequately informed girl would give up her chastity/virginity, therefore her consent just proves she was cheated by her partner who benefited from the deceit.
You are saying that no reasonable girl would violate unreasonable, oppressive norms. This is ludicrous. Clearly, the precise opposite is true: In the face of unreasonable social conditions, the only reasonable course of action is to resist and transform them.
While your statement is logically valid, given its false premise it is in fact unsound, which is to say that it is bogus.
Not the case with chess, because losing a chess game is not as harmful as losing virginity.
You have failed to demonstrate that losing virginity is harmful specifically and exclusively for prepubescent girls, except to inaccurately describe it in terms of "suffering a great loss." Such empty rhetoric does not amount to adeqaute supporting evidence.
Nor is a girl a slut for losing a chess game.
What ethical relevance is being a slut? Again, it is the norms/values that prohibit sluttiness that are oppressive and unethical, not sluttiness itself. You have it entirely twisted.
A problem with this if that if you claim serious harm, mitigated only by ignorance and therefore lack of intent of perpetrator, should sex be treated equally with other injuries?
Absolutely. Sex is not and should not be regarded as some kind of special category apart from other acts. The label appended to some act is arbitrary and ethically irrelevant; again, what ultimately matters is its consequences.
The question is - what is the damage? Mental distress, or objective loss of assets such as chastity?
While the loss of chastity itself may be objective, chastity's value as an "asset" absolutely is not. To be sure, like virtually all sexual norms/values the designation of chastity as an "asset" is culturally relative, i.e., thoroughly subjective.
You seem to be implying that the loss of chastity should have primacy over mental distress when ethically considering the two, simply because it is in some sense objective. However, both have objective and subjective elements. Whereas the loss of chastity is an objective violation of subjective norms, mental distress similarly generates objective, physiological, even behavioral correlates (e.g., increased cortisol levels, general susceptibility to disease, overtly withdrawn disposition). Your apparent attempt to distinguish the two on an objective/subjective basis is thus a false dichotomy fallacy.
Is defrauding a victim out of $ 100 000 any better than stealing $ 100 000?
That is a big "if." Please provide a plausible scenario in which a loss of $100,000 causes no distress.
PS: Given that you've expressed supportive ("the argument is reasonable") and skeptical or at least neutral ("supposedly the large loss to child") sentiments regarding the antiped position I critiqued in the OP, I'm confused as to where you stand. I composed this writeup under the assumption that you support this position; I'll just leave it as is.