I like female space marines because they piss off the chuds who think the Imperium is an ideal to aspire to
Also GW could have just said "the emperor is a big sexist pig who hates women", but instead they made up some bullshit about "only a male body can withstand the bodily changes involved in the augmentation process", ignoring fucking pregnancy as a point in favour of the adaptability of the female body. If you choose a BS excuse, people are going to call the BS.
Also even if the BS about male bodies being more adaptable were true, we would still have trans woman space marines.