It would be SOOOOO easy for them to just say that AFAB people can undergo the Spacemarinification process, you just end up with a being that is physically indistinguishable from an AMAB Space Marine.

Either invent some "gene-wash" process that tweaks literally a single one out of four dozen chromosomes, or, frankly, say there always was one and the chapters using it just didn't broadcast the fact.

It's the laziest option that resolves the problem, while also being almost kind of interesting - a warrior order that doesn't select only men, but that joining causes you to become a man.

In fact fuck it, I paint Space Marines, from now on under their armor half my dudes have non-functioning vaginas instead of non-functioning penises.