Marrying off a little girl much younger than this does seem irreconcilable to me, and to most people, I imagine. From what I have read, Muhammad married a six year old child, and had sex with her when she was nine. This is immoral in every way.
When judging a text or a situation, you make the same mistake that many make: they judge from today's point of view, without incorporating the ancient view in their judgment.
In the time of Muhammad, child marriages were common. Not only in Islam, but also in Judaism, and partly also in Christianity (on the Arabian Peninsula).
So Muhammad did nothing that was against the law of that time, as he paid a bride price for his father.
From today's point of view, this marriage between Muhamad and Ayshe/Aisha is of course a "no go"!
Today, in many Muslim countries, child marriages are made, as in Hinduism, and in some Christian sects. These women are often married to much older men for which they are the fourth, fifth, or sixth living wife. It is easy for men to divorce in Islam, but not for women, even if their husbands beat, rape, or light them with fire.
But what about Christianity?
Not as bad as in Islam, but also very bad. Consider the many cases of child abuse in churches, and the attempts at cover-up. Or the witch-burnings in the Middle Ages, on it; that women were considered less valuable in the Church, and still apply in some churches.
And what about the right of every woman to determine for herself her body and her life, which some Christian churches refuse them?
If you point one finger at others, you should remember that four fingers of your hand point at yourself!