And considerations like Deborah Gyapong's are not particularly relevant:
If Mark Steyn wrote about Muslims breeding like mosquitos rather than quoting an Imam who used that phrase, I would shun him. Human beings do not "breed like mosquitos" or "infest like vermin" as some anti-Semites have said about Jews. Yet the mosquito quote, magically removed from its quotation marks and attributed to Steyn, forms one of the complaints against him and Maclean's.
Lets say I was arguing that all Jews and blacks should be deported or sterilized, and then quoted Canadian Nazi Terry Tremaine saying something like "All Jews and blacks should be deported or sterilized." (Tremaine has indeed argued similarly, although the above is not a direct quote. Pretend it is). Were someone to then argue that I was being racist, it would not be a refutation to say that I was merely quoting a racist, because it is a standard use of quotation marks to "support the arguments of the work in which [the quote] is being quoted".
And indeed, the quotation at issue occurs at the very end of "The Future Belongs to Islam". It sums up and concludes the argument that goes before:
"We're the ones who will change you," the Norwegian imam Mullah Krekar told the Oslo newspaper Dagbladet in 2006. "Just look at the development within Europe, where the number of Muslims is expanding like mosquitoes. Every Western woman in the EU is producing an average of 1.4 children. Every Muslim woman in the same countries is producing 3.5 children." As he summed it up: "Our way of thinking will prove more powerful than yours."
...which argument is indeed that Muslims are a threat because they are out-breeding whites. Nor does the fact that its an Imam being quoted get Steyn off the hook, any more than finding a Jewish Nazi to deny the Holocaust for me would make me any less a Holocaust denier.
Quit hiding behind your quotes, Mark!