In western thought, sex is just a physical thing.
In Islam, it is physical and spiritual. We have boundaries regarding it and abiding by such laws means obey God which implies worship. You don't have this outside of Islam.
So the physical side of it is strong, it is a desire that arises at puberty. Islam does not allow for those who have attained puberty, who are of the opposite sex, to come together physically, so, by allowing marriage at such a young age this desire won't affect a person latter on in life.
A woman for example who has never had sex and is over her mid 20s or a man, out of the desire that has built up over the years, may rush into marrying any low life that comes along in order to fulfill her physical desire without taking heed that marriage is more than just sex.
In western culture, if the desire becomes overwhelming, they just commit adultery and be done with.
Allowing marriage at such a young age doesn't mean that the female is going to be turned into a baby factory, it's only a means for both parties to fulfill their desires.
And this view is well supported by the actions within western societies themselves.