The reason why most Christians argue against Islam and use shoddy reasons such as Muhammed being married to a young girl or Jihadists being the prime example of how Muslims act on their faith is because they can't argue against the fundamentals. They're too similar to Christianity.
Hm, actually I think the reason Christians would argue against Islam is the same reason Muslims would argue against the Baha'i Faith.
One thing Christianity and Islam have in common is a belief that the Figure at the center of their religion is the "last," and that shuts out the possibility of anyone else claiming prophethood, so clearly any latercomers must be "false." The logic is internally consistent, so the possibility of an actual "true" prophet for both Christians and Muslims can put them in a bind, which is understandable.
The obvious possibilities are to investigate the new claims and see if they hold water or to assume the claims are false because there is no option for them to be true. Considering the odds of any claims to prophethood actually turning out to be correct, I am really unable to fault any Christians (or Muslims) who choose not to look further.
Also, in defense of Christians, I'll say from personal experience that it's not the easiest thing in the world to really investigate Islam if you live in the West, though the difficulties are somewhat less these days.
Back before I had a family I was a regular denizen in university libraries and I can tell you from my personal experience in trying to learn about Islam that there is a lot of total junk out there, even in the more scholarly areas.
First, there was resistance within Islam for a long time to even translating the Qu'ran, so Western scholars were often depending on really bad translations. I mean, if someone is basing an argument on the Ketenensis translation of the Qu'ran, the conclusion is not very trustworthy. GIGO. There really are not any reliable translations of the Qu'ran into English until the 20th century, and even so, the earliest ones are by non-Muslims.
Would you trust a translation of the Gospel from Koine made by someone who had no interest in preserving the meaning of the text, or worse, by someone who had an axe to grind? I sure wouldn't. I think Arberry does a decent job, overall, but as a Westerner and non-Muslim if there's something missing in his translation I would be not so likely to notice.
Also, you'd be surprised how much scholarship of the 20th century was based on historical accounts that came from non-Arabic (i.e. European) sources that ultimately reach back to the Crusades. The very serious scholars will delve into Arabic sources to try and get a balanced view, but first they'd have to know Arabic and then they'd have to have access to those sources, which is not so easy.
Even in a good university library the subject of Islam and its history is pretty dicey to investigate. Much of the time you find out you're reading the equivalent of the Romans claiming the Druids sacrificed humans, and only after a lot of further digging does it become clear that you've got your hands on something that's of limited use.
So if Christians or anyone else have misconceptions about Islam, I'm not surprised nor do I assume nefarious motivations on the spot, but it would seem like the more honest approach where there are questions is to just ask some actual Muslims about it in the spirit of trying to learn, as opposed to putting on a show of gleeful misrepresentation that shows no inclination to be confused by any facts.