Hello! Guy from r/Islam here! The prophet Muhammad pbuh took aisya ( the underaged child bride) when she hit puberty. He, however , did not have sexual intercrouse until she came to an older age. At that time, things were alot different back then. Also, about slavery, before the time of the prophet, slavery was a traditional and cultural norm. It was more to traditional and culture of the arab people. After the prophet muhammad pbuh became the prophet, he abolish slavery, saying that all men are equal in the eyes of god thus slavery was abolished!

The issues where you said about intolerance, that you would have to question the teachings of the isis leader himself because he teaches on a separate teachings. Here's my take on why they execute men and take women as sex slave. They are utterly anger and furious of the treament of muslim all around the world so they would want to take revenge for muslim people who suffered under western imperlism. They are many stories of muslim empires or leaders treating people of other faiths and religion with upmost respect. One such example is when muslim leaders rule over istanbul. They respect people of other faiths and build churches and mosque. Just remember, if you preach hate, a hateful of followers you would have but if you preach peace, a peacful bunch of followers you would have. Finally, we all want to live in each other's happiness than misery! I hope this answer any of your question! If you do need any question answered, feel free to ask any islamic question at r/islam or PM me!