I'm just deeply disturbed by the fact that in Islam a man can marry his adopted daughters as stated in Surat nisa. While I understand that they aren't biologically related,, but what if they were adopted when they were babies?
Imagine a rich man who has a very large house. Out of charity he “adopts” a number of young children and houses them in his home, hiring caretakers for them. He is a businessman who travels often, so that he ends up only seeing these children once or twice a year on special occasions. Many years pass and the man’s wife dies. One of the children is now a woman of 20 who wishes to get married, and she has liked the man who adopted her on those rare occasions she has seen him, so she talks to him and asks him to marry her.
Is there a good reason for society to prevent such a marriage? It would only reduce this woman’s chance of happiness in life.
I think your problem is with the idea of a man who adopts a very young girl, say an infant, sees her every day, maybe even changes her diaper occasionally, only to go on to marry her years later when she is still young and incapable of judging things for herself. I have never heard of such a thing happening in the Muslim societies I have known. But yes, technically it could happen. And that is where culture comes in. When God does not prohibit or encourage something, He leaves it to the culture to deal with it in its own way. If you look at the many Muslim cultures around the world, you will find extremely different cultural practices among them. Islam defines a small set of rules on the most important things in life, then leaves it to the culture to fill in the blanks as they see fit. So while a man could technically marry an infant girl he brings up, it would be extremely culturally inappropriate in most cultures; most people would find the idea revolting and therefore the man is strongly pressured by his culture not to do such a thing. Note that Western laws do not forbid marrying adopted children either, they too leave it to the judgment of the culture.
Muslims do not live in social vacuums, they have to please both religion and their social norms. This ensures that by and large common sense and common morality prevail even when it comes to those things not strictly defined by religion. Islamic, instead of making adoption a matter of law, tells humans: use your common sense.
If you wish there was a law to prevent cases where a man who is a father figure to a woman is prohibited from marrying her, then you could make such a law in your own society. Islam does not prohibit making laws applying to things left blank by Islamic law. But what good would such a law do, unless we think the woman is somehow mentally deficient and incapable of judging for herself? Imagine if her adopted father is 50 is a professor at some university, while the woman is 25 and working on her own PhD degree. Is it any of our business to tell her whether it is right for her to marry her adopted father or not? Can’t we leave it to her own judgment and intelligence?
In Islam, forced marriages are prohibited and women are not property (there are of course many abuses in Muslim societies, but here I am speaking of a civilized and modern Muslim society, say a society where most women have a university degree). If we consider women proper humans, capable of intelligent thought and decision, then it is entirely their own business whether they want to marry an adopted father or not. It is only if we have a medieval attitude toward women, considering them mentally imbeciles, that the adoption question becomes a problem.
So I think the law that would clear away your worries would be a law the prohibits adopted fathers from marrying adopted girls who are still too young or who are mentally deficient. A Muslim society could implement such a law without issue if it wants to, and I am sure the majority of Muslims would support it. But why stop at adopted girls? We can have a general law that applies to all girls, prohibiting marriage if they are too young or mentally deficient. The reality is that in cosmopolitan Muslim societies this is already the practice; it is taboo to marry girls who are too young, and it would be considered rather unethical to marry a girl who cannot judge things for herself. But just to be extra sure that abuse is prevented, a Muslim society could make such a law.