I understand this is merely your viewpoint, but how can you believe that the word of God is merely available in a particular form?
The Quran which according to muslims is the final revelation to mankind was revealed in Arabic. Other holy scripture revealed before the Quran may have been in a different language. Now if you imagine Gods exact words coming in one language, then a human attempt is made to translate that into another language, you can understand that the translation will never be the same as the original. In some cases I even pointed out that the same word in arabic can have 2-3 meanings all of which apply, this won't be the same when translated into another language.
I've been told by muslims before that the English translation isn't fully relatable, and with any arguments that is always bandied out.
With it being translated by humans, it is possible there may be inconsistencies in there. Every muslim scholar from what I have seen who has refuted so-called 'errors in the Quran' has always gone back to the original arabic text first.
My problem with islam is it's inflexibility. Lots of other religions (Christianity for example) have a certain amount of flex, which is in keeping and is understanding of human nature and times.
The word of God is not subject to change over time as society changes. You call it flexibility, but what remains of Christianity today, that was there when Jesus Christ PBUH was actually here? Do you think those Christians then would even recognise these people today as Christians? Women priests, homosexual priests, gay marriages etc etc. Many things that Christianity forbids and condemns are taking place freely and openly now, and just because the vast majority of Christians convince themselves it's ok, quite a fair amount know it isnt.
Irrespective of what society tells us is right and wrong, the laws of Islam will never change as odd and strange as it may seem to certain societies but the prophet PBUH did say:
“Islam began as something strange and will revert to being strange as it began, so give glad tidings to the strangers.”
Really?
The messenger of God, starting a religion that was meant through the ages, can only be held accountable in regards the age he lived in?
Great example to have set.
Someone born in this day and age, in this society will naturally be inclined to view things like homosexual marriages as perfectly normal, yet if this person was born 100 years ago they would most probably think the exact opposite. Islam can be held accountable now as everything the prophet PBUH did was commanded by God and was an example to show humankind the correct way, so a man (of any age who has reached the age of puberty) marrying a female (of any age who has reached the age of puberty) is perfectly normal despite what you think of it now. If you study cultures before the prophet PBUHs time and even after, even if you look at Kings of England you will see this practice was considered perfectly normal for thousands of years (even hundreds of years after the prophet PBUHs time) but it is a fairly recent change that has occurred, and not by accident I should add.
Out of curiosity, what does the Christian faith say about what we would say "underage marriage"
It was perfectly normal and widespread 2000 years ago, there was no concept of turning 18 or 16 and then getting married, they followed the biological clock of a person. How about we look at what age Mary PBUH was when she miraculously gave birth to Jesus Christ PBUH as a sign from God? What age was she? She gave birth soon after she hit puberty.