Males of any mammalian species are, at least in theory, able to lactate, hence the collective noun 'mammalian', derived from the universal presence of mammary glands. In fact, all mammalian males began as female embryos before the Y chromosome was introduced into their genetic code during the absolutely earliest developmental stages of fetal growth. Adding to that, this particular quirk of nature has been theorised as to being a contributing factor to the relative commonness of biologically born human males identifying as females, whereas biologically born human females identifying as males is comparatively quite rare.
I am unsure as to the merits of said theory myself, as I personally believe there are far more factors at play than just something so 'basic'. Though to be perfectly honest, I am far from being an expert in terms of transgenderism, as it were.
Nevertheless. With all of that in mind, I am fairly certain actual futanari - or hermaphrodites, as is the more 'clinical' term - could very well be 'a thing' in the not all too distant future. Such biological marvels already exist in legions all across nature. Many molluscs, for instance, are hermaphroditic. With just a bit of 'genetic tweaking', who is to say humans could not adopt a similar style of 'genital duality', no? Hopefully, it shall happen within my lifetime, as well. As my partner would say, "Chicks with dicks are seriously fucking hot!"
~Liz.