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.
Not quite, I included the Primarchs of the Lost Legions and Omegon in the count.
The Lost Primarchs were deliberately erased from history by the Emperor for mystery reasons, and Omegon was a secret from even other Primarchs, though several of them figured it out.
Knowledge of the Traitors isn't heresy by default, but knowledge of their existence is variable by region so some sheltered worlds might not know and be outraged at your claim that a Space Marine could be a traitor and others could give you a brief history of the Horus Heresy.
Though I think the cat might out of the bag entirely in the most current timeline, like how Grey Knights aren't a secret anymore.