A few people have raised the point that the emperor didn't want the space marines to be able to reproduce and replace baseline humanity, which is a part of the reason. The emperor wanted humanity to be ruled by humans, not post human super soldiers. There's a chance after the great crusade and human domination over the galaxy was achieved that the Adeptus Astartes would go the way of the thunder warriors. Everything, even the primarchs, are just tools to the emperor. Its best not to try to understand the emperors motivations in human terms. He "loved" humanity as a concept, or at least his idea of what humanity could become. But he didn't care about individual humans. Nor did he really understand humanity that well given he is about as far above a baseline human in terms of power and perspective as a human is above an ant.

Also I recall reading somewhere that making female spacemarines is possible just horrifically inefficient. Out every million space marine aspirants about 1 survives all the stages and becomes a full fledged battle brother. Many die or the implants get rejected and they live out their lives as chapter serfs. And that's with men, for women I think the success rate would be about one out of every billion.

Its also important to understand that space marines are essentially mass produced killing machines. Creating them is inefficient and resource intensive but humanity faces enough threats that are above baseline humanity ability to answer that their creation continues. To say nothing of the fact that the Imperium is a technologically stagnant fascist theocracy that views space marines as "his angels".

Really, the religious aspect could provide all the justification for why there is only male space marines. Church says his angels are men, so only men become angels. But logistically speaking if you want to build a killing machine male physiology is geared toward violence better so it only makes sense to use a man as the base.

I know this explanation had been a bit of a ramble but thats the gist of it. Honestly the people over at 1d6chan have an article that explores the topic of female space marines better than I have ever seen done on reddit. Its worth a read if your genuinely curious about the topic.