I think that a lot of the ambiguity or lack of detail regarding this subject matter is intentional, as to allow Referees and players the latitude to adapt it to their own tables. I believe a lot of the '''gaps''' in Charted Space lore are, in fact, precisely that; what exists is a scaffolding, with the walls and details left for us to fill out via the beautiful art of Interpolation™.
That aside; from all I have read about the Third Imperium, I find it extremely hard to believe that there would not be inter-species relationships, even between members of humaniti and other species. I think especially in high population, cosmopolitan worlds you would see it with enough regularity that it would be unusual but not weird, and certainly not unheard of. But I do not believe this would hold equally true across the entirety of Charted Space – attitudes regarding it very well could and I believe should be varied depending on world, region, polity and species involved.
The Imperium itself might not give a hoot, but individual member worlds certainly might. In Regina a human and an Amindii building a life together might be seen as unorthodox but entirely fine, perhaps even admirable. In more backwater worlds though, this would likely be seen with repulsion, perhaps even disgust.
I'm inclined to believe that in the "default" culture of the Imperium, that is, that hazy zeitgeist that is carried between its worlds via traders and media and the nobility, where you're so accustomed to seeing different sophontic beings that you stop associating the concept of 'people' with definite shapes and sizes, the notion of inter-species relations wouldn't really be seen as something to write home about.
But I do think the influence of the nobility, and especially the hereditary nature of nobility titles, would create a soft, unspoken pressure and expectation that people should stick to their own species. 'Oh, you're getting married? How joyous! Who's the lucky suitor?' 'Oh, a Llellewyloly... I see. Well, I'm sure you'll be very happy together.'
Among actual nobility, unlike the common citizenry, it might even be more clearly frowned upon and disencouraged behind closed doors.
All that said, I do think that we should probably address the elephant (well, actually canine) in the room though: I will readily admit that the notion of a human being a relationship with someone who is essentially a walking, thinking, talking dog is very weird. I, personally, am not sure if I want to think about the implied consequences of what that relationship would entail (heh. Tail).
But I'd argue that this is precisely the merit of leaving all of this unspoken in the subtext; I might not want to think about it but someone and their table might. It allows each individual table to engage as much or as little as they want with it, to the extent of their comfort with the subject. I find this approach commendable.
On the subject of comfort zones though, personally, while I would not want to explore the more... physical... aspects of interspecies relationships between humans and other sophonts, I am actually quite interested in the emotional and romantic components of love between them and the narrative potential it affords. If you'll allow me, consider the following scenario:
Archduke Brzk of Antares has a duty to his people to sire children to carry on his title and legacy. He has taken for his wife the hand of a lady from one of the few noble Vargr families, but their relationship is cold at best. They carry out their respective expected duties to one another, but that is the sole extent of their interactions.
Most of Brzk's time is spent instead with one of his aides, a human lady. Unlike with his nominal spouse, with her Brzk feels like he can truly share his thoughts, his interests, and most refleshingly of all, let down his guard and drop the mask required of the nobility and just be... himself.
He often thinks back to that evening in the gardens where they stopped 'neath a tree and sat, talking for hours, and how as the setting sun bathed the three peaks of Gaakish, he laid his head on her lap and, for the first time in who knows how long he told another living soul of the things that burdened him and weighed down his chest, and how she listened, her eyes unjudgemental, without requests or demands or expectations – just warmth and– and true caring.
And he wishes that he could spend his time with her, relive that moment every day. How would he like for them to grow old together... but he is bound by his duty to his people. He could never do something so selfish.
And so up the mask comes again, and on with his duties he goes.
Maybe I'm just a wobbly-hearted, sappy fool, but to me this kind of stastruck love narrative is just, mhm-hm. I'll be having some of that at my tables, yes please.
