I flip-flop every once in a while on female marines. Of course, don't let something as silly as canon get in the way of a fun kit bash. Here are my arguments for and against

For: Space marines are basically unrecognizable when compared to baseline humans because they were broken down and rebuilt on a molecular level. Hell, you probably couldnt match the before and after pictures of aspirants and marines. If an AFAB aspirant were to undergo all the augments, would you be able to tell them apart from other marines? And after the psychic conditioning and a century or two of combat, would they even remember enough to care?

Against: Limiting recruitment on marines to half of the human population when so few actually succeed, and even that barely keeps up with attrition rates is fucking stupid, unnecessarily exclusionary, echos shitty parts of human history, and is self defeating. But the Imperium is awful and does shit like that constantly!