Studying English in school as a second language, I learned that human being would be the proper noun to describe a member of the Homo sapiens species, but it seems human is perfectly acceptable in English nowadays. It can even sometimes substitute (supposedly gender-specific) man where that was used in a gender-neutral way, e.g. in humankind, but not in *chairhuman, *firehuman etc.
The gender nouns man and (derived) woman, however, have their plurals as men and women, respectively, so why is umlauted *humen not acceptable at least as an alternative to the regular plural morphem +s which is used to form humans? The same question applies to some peoples or tribes, e.g. the Germans and Normans, not *Germen and *Normen.
The question even applies if human and man have different actual etymologies (Romance vs. Germanic), because perceived or folk etymology is often at least as important.
Also, does this effectively render these words gender-neutral, although they seem to contain the gender-specific morpheme man (in singular)?