Oof. Sorry but you are absolutely not up to date with grimdank memeology.
The eldar woman is Yvraine from craftworld Biel-Tan, she is a prophet for the new eldar goddess Ynnead and goes around the universe trying to find new followers. She's in a sect, in short.
She stumbles upon Belisarius Cawl and helps him revive Roblox Gullible (nobody says Guilliman, you fool).
From that simple thing, people have gotten quite creative in terms of dreaming a relationship between Yvraine and Rubik's Guacamole. Yes many rule34 depravations have naturally followed.
To sum up, in a world where every major hero from 30k has more or less disappeared, the romance of Yvraine and Robust Guitarist is the only chance to repopulate the galaxy with hope and beautiful babies
Default Space Marines were made infertile, but the Chaos Marine Fabius Bile apparently undid that for some of his experimental creations and he's his whole thing. Also, they might not be infertile at all, but just asexual.
Also, mutation of gene seed over 10k years means nothing is perfect so it could, in theory, just not work in various Loyalist Marines. A Space Wolf, Lukas, may or may not have had kids after becoming an Astartes.
It's mostly irrelevant though, because Guilliman there is not a Space Marine.
He's a Primarch, one of 21, and we still don't know everything about their creation so it's entirely possible he's fertile. Or maybe not, because he was custom built by the Emperor and he didn't want Astartes replacing humanity, much less Primarchs.
But the lady is a Space Elf, who have weird technology. And she spent some time with the BDSM Space Elves, who have even weirder technology. And we know Human/Eldar hybrids can be born. But Guilliman, as mentioned, isn't a typical human or Astartes. But he's full of Warp Fuckery.
So, really, the answer is "it would work if an author wanted it to work"
Oh 21, how interesting. Can you give us those 21 names then? No? Just 20 of them maybe? Still no?
Is it because you made that number up as everybody knows there has always been 18 primarchs? Any other number is quite silly.
The "Happy Father's day" card was a surreptitious "I'm pregnant" announcement that first wasn't understood by Guilliman because he is figuratively called the (gene)father of the Ultramarines and their various successor chapters.
This was then clarified with the pregnancy test when drawn/photoshopped like that is a common meme among animé circles.
In the fourth and fifth panels Guilliman has a moment of shocked understanding as to the meaning of the letter, and a psychic projection of the God-Emperor (essentially Guilliman's Father) showing disapproval.
Now to clarify:
The crowned Aeldari woman is Yvraine, emmisary and leader of the Ynnari (a mixed group of Aeldari who are attempting to bring back Ynnaed, the Eldar God of the Dead as a means of countering Slaanesh). The Ynnari had a whole thing near the end of Seventh Edition 40k in 2017 where among other things they allied with the Imperium against the 13th Black Crusade. Amongs that allyship, Yvraine with her mastery over death decided to bring Roboute Guilliman, the 13th Primarch, out of stasis with the help of a specially designed armor, the Mechanicus, and the Inquisition. Guilliman was downed and put into stasis on Ultramar some 9000 years prior after a fight with Fulgrim and had poison in his system. The armor and intervention of Yvraine let Guilliman finally be an active player in the setting where he did various things like become Lord Commander, have a meeting with the Emperor on the golden throne, start the Indomitus Crusade, and in various ways advance the setting a bit (the other big change is that Cadia has broken and a great rift in the warp has split the Galaxy in two). Not much has happened to the Ynnari since 2017, as the reception to the line / Army wasn't enthusiastic and they even had a lore book cancelled. In the three editions since they just quietly made Ynnari not its own army anymore as they were just three new models and a headswap (or just palette swap) on some troops, and people didn't like the "soup" aspect of just mixing in various knife ears together.
The internet being the internet, people saw that Guilliman and Yvraine were in the same room and concluded that they must be scoodilypoopin' because opposite gendered people can't ever just be friendly. In this fancomic, Guilliman is the "dad" of Yvraine's implicitly announced unplanned child. The Emperor would likely be disapproving, as abhoring the alien is one of the core tenets of the Imperium. That said, in old canon there was indeed a half Eldar Ultramarine Librarian so hybrids are officially hypothetically possible, but that's retconned out. I think Aeldari have a described triple helix DNA or something like that.
By the way this comic would normally be not allowed here and directed towards !grimdank@lemmy.world , but because it’s being framed as what appears to be a genuine question I will make a one-off exception.
Nothing against memes/humor/comics or the people wanting to post them but if they are allowed generally they will overtake the other content.