First you've said: "you sure wouldn't catch
Allah having sex with grown men". Allah (الله) is the Arabic word for God, used by Arab-speaking Muslims, Christians, Jews, etc. Muslims are particularly careful to avoid depicting God as something that can be understood in human terms, so it would be an impossibility for their image of God to have sex, father children, kill himself, and so on.
Now you're talking about
Muhammad (محمّد) - exemplary though he might have been, no one is claiming that he wasn't human (i.e. son of Allah, incarnation of Allah, etc - none of that). Furthermore, Muhammad was born into a society where polygamy and child marriage were common, and the religion he invented (yes, invented) actually limited and discouraged those practices. Sexual relationships with young girls have been considered desirable in most cultures up until the 19th century.
Anyways, where were we... Oh, right: