There is some genetic transfer between the space marine and their implants, geneseed has qualities that vary depending on the lineage it's drawn from. Space marines sometime inherit physical or psychological qualities of their predecessors and can in turn pass down their own qualities. Obvious thematic reasons behind this aside, it means they'd probably have gametes corrupted over time by their implanted genetics. From a thematic standpoint it'd suit 40k if Astartes sired offspring were born deformed and wretched as part of the common theme of genetic tampering being dangerous.
I guess some genetic material could somehow get into the kid, but it could not be advantageous, and even if, certainly not enough to create a full-on Space Marine.
Essentially.
But I think the closest we'll get to a proper answer is that the RPGs gave them Chem-Gelded (which also represents mental discipline against temptation mind you), and whenever the concept of love is mentioned with them in novels it's familial, so I think sexual arousal just doesn't happen to them. They're too distanced from normal humans to feel it, along with a bunch of other stuff.
I know this is somewhat of a weird line of reasoning, but I mentioned that there doesn't seem to be many homosexual (or maybe they're all Slanneshi now) Space Marines (it's been thousand of years, they form tight bonds, they probably walk around half-naked a lot, it's pure law of probability, some of them would have to think about it one day or another if they felt arousal at all), nor any of them really exhibiting any form of sexual interest - the technical fertility of the, well, seed, is up to debate, but it is pretty obvious they don't seem to feel it at all.
Of course it's not really necessary for some form of romance to have arousal and all, but at that point it might be indistinguishable from regular camaraderie for them, and they don't exactly have a lot of chances to get that going on with females.
Then there are Space Wolves who seem to be still attracted to females, although still doesn't really explain if their equipment actually works, so to speak, since I think all instances of actually landing in bed can be explained as going at it before becoming a Space Marine.