Co-authors request that others do not use "he" as a pronoun - is this reasonable? We are a bunch of people writing a collection of papers. Two of the writers are from Iran (where the common language does not have gendered pronouns), and they have expressed to me some concern with regards to the lack of gender-neutrality in our papers.

As you may know, English writing often exhibits some degrees of gender-bias, mainly the use of "he" to refer to a person with an unspecified gender. These co-authors would like to avoid such bias if they are to put their names on the papers. 

They've told me this as we are close, but haven't talked to anybody else (there are 3 others). Is this a reasonable request? What's the correct thing to do here? I could just ask the others, but what do I do if somebody gets offended by the request and refuses? I doubt it'll happen, but in the current public discourse where "political correctness" is the new pejorative buzz word, I am not so sure.
This is an entirely reasonable request and you should honour it. It is very easy to write English in a manner that avoids the unfortunate use of 'he' as a placeholder for either gender, and doing so avoids needless sexism in your writing.