I have long wondered how people think about marriages or love relationship with with member of another Race in Third Empire. Sure, everything varies from planet to planet in Traveller. But how it is generally in those sectors where the influence of Imperial civilization is strongest?
I would suspect that in the Imperial core (and especially in the Solomani Sphere) inter-species relationships are at least frowned upon if not actively persecuted. Certainly between non-genetically related species. I suspect even the more morphologically divergent Humans would be "fair game", and relationships between any members of Humaniti would be accepted in most parts of Charted Space. (Again, maybe not in the Solomani Sphere....)
Out in the Marches and unincorporated areas, I would think the rules would be less standardized, though romantic (or at least physical) love between non-genetically compatible species would IMHO be uncommon, since it seems unlikely to me that, for example, a Bwap and a Vargr would find each other attractive. (They might become the best of friends, in the "opposites attract" sort of way of course, but we're talking about "love & marriage".)
Inside other stellar polities, it would depend. I personally believe that all species would tend to favor their own naturally and that there would always be exceptions to that rule. Each "state" would decide what to do with the "deviants" on it's own (re-educate them, leave them alone, suppress them, etc.)
Zhodani - my suspicion is that they would have a "Humaniti on Humaniti only" policy without any regard for what breed of Humans are involved, and anyone found to suffer from bestiality would be re-educated.
Vargr - Almost no rules at all (as typical for Vargr), though I suspect that most Vargr would feel a little funny about getting romantically involved with "naturally evolved" creatures. (Though there's always the whole dog "leg dry-hump" thing to refute that idea, LOL)
K'Kree - definitely not, and they'd "encourage" the lesser species in their domain to behave themselves as well.
Hivers - definitely not. But then again, I don't think they even have the concept of marriage and love in a way that makes such discussions meaningful.
Vegans - Can't see it. (And really, who'd want to? :grin
Aslan - Strangely, this is one of the few I can almost see it with. With the caveat that since Aslan seem to associate "Sex" with "Job" and they sort of see all Humans as looking alike, they might get confused and fall in love with a male Darrian Engineer, for example. But at the end of the day, I don't think it would work out in general - clan and appropriate gender roles are too important to Aslan psychology, and even though some outcasts might go for it, I can't see any clans allowing inter-species relationships.
Darrians(Yes, they're genetically part of Humaniti, but their culture sets them apart) - They're demonstrably willing to pair with non-Darrian Humans, but their stoic and ordered way of looking at things means to me that even were a Darrian to fall in love with a non-human, they'd never let those feelings show.
Droyne - Not a chance. They don't even sit down to eat without consulting the coyns.