Sexually mature doesn't mean emotionally mature.
A girl might be sexually mature (ie physically able to bear a child) by 12 or even younger.
In Islam, it is considered "moral" to marry a girl that old, and have sexual intercourse with her. Who are you to prove the Muslims wrong? It is all about perspective.
All I know is that God himself created woman to be able to bear a child at aprox. age 12. if God wouldn´t have wanted woman to start having sex that young, he wouldn´t have made them able to even have multiple orgasms at that age; not to mention having a child, and
possibly even a healthier child than an older woman can have.
Who are you to contradict our biology?
On the other hand, I know that a woman that old cannot have an emotional maturity to be sexually active; but that is were sexual education (from parents) comes in. A woman is perfecly capable of saying "no" to this sort of things, if she doesn´t want to say "no", well that is another subject.
I know many woman who didn´t start having sex until older, and that was their conviction not the men they were dealing with. I know plenty of men who hitted on this girls when they were younger and wanted to have sex with them at an early age; the girls were mature enought to say "no". If a woman doesn´t want to say no, and you don´t want her to have sex, well, you might as well cage her in prison before she starts doing acts you wouldn´t "approve" at all - lol -
This applies to men as well (girls at age 12, boys at age 14), but without the "child bearing" part
.
Now I know the moral implications of this, many people would consider this a "heresy"; I just don´t freaking care you know? I´m just saying the truth, the truth doesn´t have to transform itself to fit your standards, you are the one who needs change if your standards don´t fit with the truth.