As-Salamu 'Alaykum
Bismillah Ar-Rahmaan Ar-Raheem
I'm going to conclude this by following points: 1 - Whether modernist or liberal humanists, their objections against laws of Islam, whether child marriage or flogging an adulterer, are based upon modern standards/principles and their doctrine of evolution in morality. In layman's terms, they believe that there should be evolution in morality. However, in order to prove their case they need to prove 2 things: a) that there SHOULD be evolution in morality and b) we in fact have morally evolved for better. Until they can, and never will be able to, prove this, their views remain morally bankrupted and oppressive to societies.
2 - For example in case of child marriage they say that our norm have changed and people are not mature, etc. They deem or regard Islamic ruling as unfeasible now because the societies have changed and morally decayed within last century. Due to the compounded ignorance of modern societies, the age of responsibility rose from 10,11,12 to 18 and even at 18 many of them do not learn responsibility until much later. For not preparing people at early age to take responsibilities is nothing but oppressions, and an evidence for this is social problems within our societies.
3 - Again, regarding child marriages, why one should follow modern inconsistent laws/norms? Their flawed methodology and reasoning can be refuted by disagreement among themselves and contradictory laws. In one country it is moral to marry a 14 years old girl/boy but in another country it is immoral and crime, and he/she will be labeled as 'pedophile'. How can anyone follow such inconsistent/contradictory methodology? and Allah knows best
Because you cannot force your acceptabilities on others. Marriage at an early age is acceptable and encouraged in Islam but its not a law that you have to get married before this age. How many girls you know in your family who are ready for marriage at 12?