"The idea here is that virginity/chastity is an expensive long term asset, like a house or education. In case of sex, it is assumed that the child suffers a great loss, and the adult benefits. The fact that the child consents merely proves "fraud" - 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."

Virginity/chastity could be said to be a long-term asset, since men prefer a female who will not potentially cuckold him. However, your second statement is not true. Many girls would still give up their virginity even after being informed of the loss to mate value. Apart from this, and perhaps more significantly, your argument applies not only to girls under an arbitrary age of consent, but to those over it as well. Females over 18 (or whatever the AoC is in your country) also suffer a loss to mate value from giving away their virginity. In fact this argument has no age dynamic and is reminiscent of the old way in which sex was regarded; age is not relevant, but marital status. The key thing being that a women is married before she gives her virginity. Once married, loss of virginity no longer matters because ideally she will go with no other man, and consequently her virginity is no longer an asset.

Furthermore, it is not clear that a man taking a girls virginity would be fraud. Your argument rests on the notion that chastity is an asset and one that ought to be appreciated by a girl, otherwise she mustn't be informed. Except that many girls will not apply quite as much weight to this as you do. What you are doing, is insisting that a girl attribute as much weight to this fact as you do. Facts may be facts, but individuals should be free to attribute as much weight to them in their decision making process as they wish. Your argument implicitly seems to deny agency to individuals. A man taking a girls virginity on the promise of marriage and then reneging on that promise would be fraud. A man taking the virginity of a girl who is unaware of the loss to her mate value, or does not attribute as much weight to it as you think she should, is not fraud.

"If the less-informed party suffers a large loss and the better-informed party a modest benefit, this suggests fraud by the better informed party."

Except that as you say, many people will be as ignorant as one another; fucking without a care in the world, like our great-ape cousins. Anyway the main problem here is that this does not suggest fraud. How are you defining fraud? I would define it as such: "Fraud occurs when one party induces another to consent to an act under false pretenses." As as simple example: A boy tells his sister that if she tickles his back, he will reciprocate. She agrees, and tickles his back. In the event, the boy does not reciprocate, and runs away. This is fraudulent (not worth half a lifetime in prison though haha). Is it fraudulent when a car salesman sells a car to a customer that he knows is not robust, and will have alot of problems in a few years time, while the customer does not know this? Of course not; the onus is on the customer to do his/her research when buying a car. Even if he was aware that he would be given trouble in a few years time, he may not attribute much significance to this fact. That's were individual agency comes in. Someone like you cannot say, "I really think you should attribute more significance to this fact." It is up to the individual what they do with the facts, once they know them.