Astartes among loyalist forces are typically recruited young.
For the majority of chapters these space marines are recruited before puberty or during it. Therefore we can say with some degree of accuracy that your typical Astartes is socially retarded around women, and furthermore is probably a virgin.
Some stories touch on exactly these points, with sisters of battle lusting after some blood angel or ultramarine who just doesn't realize it, or a human female will flirt with an astartes only for it to be taken the wrong way.
As part of the process of becoming Astartes involves one's testicals being blown up inside the body, and the production of testosterone being then produced by some other organ, we can theorize that a Space Marine has no libido.
The sons of Russ, the space wolves however do things differently. They recruit solely from battle tested warriors, who are typically in the 20-30 year old range. These men have already lived a life of strong alcoholic drinks, and hundreds of loose women. And so they are the sole Legion to actually flirt with girls, that being said it is debatable whether they are even able to actually go through with the act even if an opportunity presented itself.
As physically they may lack the ability to even get an erection. Certainly there has never been a recorded instance of an Astartes with a boner.
So Astartes don't have sex ever, but its not a big deal because they never did to begin with. They have no idea what they are missing and so never feel it's loss.
Contrast this with the Emperor's Children. A rebel legion which worships Slaanesh and are known to rape people to death.
In fact so wild about rape are they that if there was ever anywhere in the galaxy that produced artificial cocks designed to attach to stc dreadnaught designs, and hook up artifical nerves to the inhabitant of said dreadnaught. Well it would be them.
Did their apothecaries make Astartes Viagra? Is it simply a mutation? The only way to know for sure would be to conduct a thorough analysis of a captured EC legionary.
But who would conduct such a test? And for what reason?
And here's where the whole thing ties together.
The Space Wolves. They alone miss fucking, and they alone are known to get drunk off a special homebrew they make.
I posit that at some point during the ten thousand years from m31 to m41 a Space Wolf squad or possibly company got really drunk, told a bunch of stories of sexual exploits to each other, and then started bemoaning the last couple hundred year dry spell they had experienced since becoming Astartes.
Then in a drunken stupor (and accompanied by a couple dreadnaughts looking for robo dicks) they embarked on a sort of panty raid of an EC vessel. Using a boarding torpedo assault they land simultaneously in the EC apothecary, and it's machine shops. After locating enough space viagra and robo dicks to last the next couple decades the two forces fight towards the hangers, commandeer a thunderhawk, and fuck off back to Fenris, where they then go on a month long drinking binge and brothel visit tour, which takes them all across Fenris.
Logically it must have occured at least once, and is possibly a semi-regular thing.
Maybe there is a special enmity between the two legions, a hatred from the EC over those goddamned furries who show up and steal all the party favors.
Idk. Just some thoughts.