I believe space marines don't have sexual urges?
Which leads me to the question... How do they stay so aggressive without testosterone.
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So first up, I suspect the downvotes are more than likely due to lack of research. Searching for 'space marine sex' online there are a number of results and the fifth result down provides a link that explains:
“...He barely notices them. Sexuality is a forgotten concept, alien to his mind, merely one of ten thousand humanities his consciousness has discarded...”
Perhaps there is another reason, but that is my assumption. The second part of your question has more legitimacy in my opinion...
So, regarding Astartes emotions they do of course still have them. Lukas the Trickster is always my clear counter point to anyone that says otherwise.
So the next question is what do they feel. Over the years the space marine has evolved from a mindless warrior monk into the super humans we know and love today, but each and every one of them goes through some serious mental reconditioning.
Space marines are strapped into what I always called 'knowledge machines' that teach the warrior all the things they would need to know to operate effectively. Thankfully I think GW realized this omission and in the new Dark Imperium series (I believe the second book) they named the devices as Hypno-mats. During their hypno-therapy they learn High and Low Gothic, what an Ork looks like, how to maintain and use their weapons, the Imperium and chapter's histories, etc.
At the same time while they are strapped in, they also have their minds played with so that they suppress certain emotions, chemicals, and desires (like fear, sex, etc.) and other profiles are boosted (like honor, tradition, etc.).
And while not all reconditioning is the same from one chapter to the next they all follow very similar tactics to maintain the same basic result: A warrior that knows everything he needs to and that won't turn tail when death and defeat seem imminent.
Additionally, as an aside, there are absolutely no instances of a space marine having sex in a GW/BL published work. They don't do it. There are a lot of rumors of it actually happening, but unless you get someone to get you a picture of the actual passage from a book with a title, chapter, and page number to prove it don't believe it. I could go into that more, but it isn't your question...
Regarding higher aggression and testosterone there is no proof really going one way or the other. However, I would argue that it is not necessary. They are not mindless brutes in battle, but intellectual warriors. While their battle gear and bodies both have the ability to inject them with chemicals like pain-killers and etc. none* really do anything to boost their aggression/testosterone as far as I am aware. The Thunder Warriors were the berserkers, not the Astartes.
*One of the new implants that goes into the Primaris line could be claimed to do this in a way. The Belesarian Furnace is a new implant that injects combat stimms into the body when it suffers severe trauma or stress. Not exactly the same thing, but...
Finally, we don't know exactly everything that goes into the process of creating a space marine. We know a lot mind you, but not every single step, chemical, and alteration that is made to the aspirant. The Astartes are test tube experiments that are shot up with all manner of chemicals and genetic modifications made to their human forms to create the super human beings they are to become. It is entirely possible that the gonads in space marines are super charged to create more testosterone for the warrior moving forward.
For start their biology is hugely different than one of a regular human male, even their blood is different than human blood - it is not a huge stretch then to imagine that their body reacts differently to testosterone on a biological level. After all, the Emperor wanted his best soldiers to fight and kill enemies rather than be a raping horde - such behaviour would not only tarnish their image as "angels of mankind" but also directly push them into the embrace of Slaanesh. Thus, he made them asexual by design.
Then there is conditioning: the candidates start usually as children or young teenagers and are heavily conditioned to focus only on combat - in The Emperor's Gift novel an Inquisitor retinue takes a shower together and one of the female members is quite interested in a young Space Marine, who completely doesn't understand her advances:
On more than one occasion I’d stood with her and her warband in the communal showers after training, blind and numb to any sensation of desire, watching her wash her hair and listening to her speak of serving in the sweltering jungles of Voroxis, killing heretics who spilled from a downed rogue trader vessel. Bizarrely, the Khatan had declared, upon seeing me wash myself, that my ascension to knighthood was ‘a great shame’. Captain Castor had explained the meaning to me several months later, though I still didn’t see the humour in it.
Now, there are of course exceptions: there was a mention somewhere that Space Wolves like to dring "Mjod" - a Fenresian mead - which can cause them sometimes to look for a female company, but that might be just rumours (i.e - retconned). And there are of course Chaos Space Marines following Slaanesh, who certainly engage in all possible perversions.