From reading the posts so far in my opinion the problem with the people who seem to be offended by this law seems to be their tendency to look at the issue with prejudice.
This prejudice is evident by the very title of this post: "Just marry her". This very statement presumes and tries to falsely relay to the reader that the adopted child in question must be a female and more importantly she will have no say in the matter, "just marry her", like who gives a fuck what she feels or thinks you just do to her whatever you want! Is this really what the law is saying?
Does this law allow for a man/step-father to marry an underage girl? No. In that case if there is going to be any marriage would it be between two adults? Yes. More so would the marriage be between two consenting adults? The answer is also presumably yes.
Furthermore, the extreme of case is considering a child that was adopted as a toddler and the man raises her just like his own biological daughter and then more than a decade later his love for her changes to another form and her love for her "father" also changes to another form and they both decide to marry each other! ! But how about the other extreme cases where someone like Woody Allen adopts a teenager never really identifying himself as her true father but more so as a supporter and caregiver and couple of years later when she becomes consenting adult they both fall in love and want to get married?
What if a secretary falls in love with her boss who is very nice and generous to her and they both decide to marry each other? Is that considered sickening and immoral? It would be if the boss tried to coarse, threaten or force the secretary to marry her. But what if the feeling was mutual and they both were consenting to the act? How would that be morally any different than for example the case of Woody Allen mentioned above?
Now the above argument I made has nothing to do with “sharia” laws but is rather simply based on legal and ethical argument. As far as the “Sharia” is concerned however, while Islam, Koran and Prophet strongly encourage fostering orphans (as evident by Muhammad’s own life) it strictly prohibits adoption of children in the current “Western” style where the adopting parents would identify themselves to the child as his/her real parents.
“Allah has not assigned unto any man two hearts within his body, nor has He made your wives who you declare (to be your mothers) your mothers, nor has He made those who you claim (to be your children) your children. This is but a saying of your mouths. But Allah says the truth and He shows the way. Proclaim their real parentage. That will be more equitable in the sight of Allah. And if you know not their fathers, then (they are) your brethren in the faith, and your clients. And there is no sin for you in the mistakes that you make unintentionally, but what your hearts purpose (that will be a sin for you). Allah is Forgiving, Merciful.” (Al-Ahzab: 4-5)