Fucking FSM discussion, here, of all the places, and it seems to be three pages deep already, fuck. Ugh, why? The whole thing reeks of real life politics and the whole deal of "male CIS scum" and "tumblr feminazis" and I would love if it stayed outside of goddamn Warhammer.
1) Malcador The Sigilite actually joked to The Big E about making female Primarches, but he said it would be too much additional hassle for not enough gain. He also was worried about possibility of love-related problems. I mean, what if Horus Heresy was not over Chaos bullshit but rather over a woman? And while, yes, The Shamans making up Emperor were not enterietly male and he could supposedly pick a female form, it would be additional hassle. There is also this thing about Emprahs true form, which may or may not have been genderless.
Anyhow, this explains why Primarches were male, and if primarches were male, it explains why Space Marines must be male. The gene-seed is not Emperors man-sauce per se, its actually genetic trace of respective Chapters primarches with a bit of Emperor diluted through thousands of years of Marines (yes, Marines do grow two organs containing Gene-Seed, though skilled Apothecary can harvest enough genetic makeup for more than two Marines). Space Marines candidates are picked to resemble the Primarches the most genetically, so being a woman kind-of-disqualifies women. Also, the whole implanting proccess is geared towards males, so while it could proably be done if there were female Primarches (missing Legions, anyone?), without Big E coming back and deciding "lets have gurl mareens", it is fucking unlikely. As for the Marine sterility - no canon word of that, but supposedly yes. Wether their junk works or not is up to fanfiction youre reading, though.
2) There is a joke showing an image of Space Marine titled FSM. There is no way to really distinguish a FSM from regular MSM, especially in armour (although, during Rogue Trader there were models of "Female Adventurers in Space Marine armour", but we do not talk about those), which is actually why the only "canon FSM" was not discovered (well, she was, but due to differences in fighting style from the supposed owner of armour, not due to sudden apperance of boob plate or something). So, no Sexy Marines for you, just huge blobs of muscle in metal armour.
3) If someone is saying that FSM should be a thing because thats sexy and the only reason they arent is some sexuality problems among the fans or creators... fuck off. This isint your personal fapfiction.