Islam is purely a religion based on the Koran. You can only become a true Muslim, a follower of Islam only when you accept and follow the teachings of Koran. Followers of Islam believe that the “prophet” Muhammad was their true prophet of God. Muhammed married a 6 year old Aisha and slept with her at the age of 9 according to Islamic teachings.
Image is from Sri Lanka, a rally organized by an extremist group called Tawuheed Jamath who demanded their right to marry underage girls. Link to the story here > https://www.colombotelegraph.com/index.php/sltjs-mmda-protest-and-un-islamic-abusive-criticism/
Marrying a 6 year old and having a sexual intercourse with a 9 year old is something no one in the civilized world would approve. Its considered child abuse, and such individuals are called pedophile. Every democratic values, human rights and child protection laws will consider that as a criminal act.
Now the Muslims who say that they are supposedly against the child marriage culture in their culture must take an honest stand between their stance on child marriage and the Quaranic teachings.
Do Muslims who are against Child Marriages don't agree with "Prophet Mohammed" ?
Any logical person would say that marrying a 6 year old is wrong and having sex with a 9 year old is also wrong. If so the Prophet Mohammed was also wrong. But then comes the issue that anyone who DO NOT believe in Prophet Mohammed is not a follower of Islam according to Islam.
Therefore its very simple. if a Muslim honestly against Child Marriages,
1) He/She should be against Prophet Muhammed too. or,
2) That he/she should question the authenticity of the Koran
3) If so, must agree that the Koran teachings don't fit in today's civilization,
Either way, one who disagree with child marriages and Islam teachings on same cannot continue to be a follower of Islam due to reasons mentioned above. Its not just child marriages but dozens of other issues handful of Muslims now starting to question. For instance Muslims who have become atheists and those that have written controversial books questioning Islam.
But if Muslims say they are against Child marriages and still believe Prophet Mohammed's way of life is their religion is no way an honest advocate against child marriages. Because, for someone to have complete faith in the Koran and be vocal against child marriages is not possible. It has to be either one of them.