There is a defence of Muhammad- the founder of Islam- in the Guardian, with regard to accusations that he was a paedophile for marrying a six year old girl.
A lot of effort is made to contextualise this by comparison with what was normal at the time- and it is true that taking very young wives was a lot more acceptable in earlier centuries than today. Various figures from ancient times to the late middle ages did this.
This kind of context matters in judging most historical figures but the problem is Muhammed is not most historical figures. If Mo had simply been a particularly successful warlord it would be tiresome to denounce his sexual morals or the fact he was a slaver. However he and his adherents have never claimed that he was simply a man of his time, but rather that he was the source of eternal truths, that are as true today as they were then.
It is similar to how the massacre of the Trojans by the Greeks is not a problem for modern admirers of ancient Greece whereas the massacre of the Canaanites by the Israelites is a problem for those who believe that the Bible or Torah are divine sources of truth.
Of course in the modern world most Christians and Jews don't endorse the worst aspects of their holy literature. This is not the case with Islam, when simply speculating that some passages may be allegorical is a death sentence in much of the Middle East.