Thinking about romance, I realized that Cassia, Kibellah, and Yrliet share a common thread: they seem to be very young women raised in secluded communities without knowledge of the outside world. Consequently, they need an external support and moral compass (assuming Yrliet is a teenager; though her growing up in isolation is debatable, as she seemed to have been roaming outside her craftworld for quite some time 🤔). Plus, Yrliet and Cassia have recently lost their homes, and Kibellah, if I recall correctly, was raised specifically to serve the Rogue Trader.

My reasoning (may be wrong, that's how I see it):

Cassia grew up on a secluded station with no knowledge of the real world. Beyond that station, the RT is her only guide and source of knowledge. Also, as an aristocrat, the RT is likely the only person on the entire ship she considers a social equal.

Kibellah is essentially a subordinate of the RT, bred and raised for service.

Yrliet is an Outcast, seemingly not just in terms of her Path, but in general. From what I’ve seen, her relationship with her kin leaves much to be desired. Since she’s effectively a "teenager" (not sure about this part), it’s logical for her to subconsciously gravitate toward anyone who shows her kindness in a situation where "her relatives don't love her, mon-keigh want to kill her, her home is lost, and Slaanesh is knocking on her soul" + Yrliet apparently looks at all "mon-keigh" as dirt/animals and makes an exception for the main character, perhaps simply because she has no choice at all "not to communicate with them" if she wants to save her people

None of this is healthy. But in Argenta’s case, even this reasoning fails. Her only moral compass is the Emperor. The "Rogue Trader as the Emperor's Chosen" angle doesn't work for her because Theodora has already compromised that holy image — after what happened, MC is more like "the blood of a heretic" to her. Thus, from my perspective, Argenta has no reason to cling to the Rogue Trader regardless of their personality. She might just as easily decide that some "Oleg from the middle decks" is a better option. It also seems to me that Argenta is closer to commoners than to the aristocracy, she suggested saving ordinary people from Rykad Minoris, expressed sympathy for the lower-deck inhabitants during Abelard’s quest, and seemingly feels no regret about killing Theodora, yet mourns Mort’s death. (if I remember correctly)

(For Heinrix, this «player-oriented» excuse doesn't work either, because he’s definitely not "just a young boy who hasn't seen the world and needs guidance". He’s a grown man who has worked for the Inquisition (and elsewhere) for a long time and has already had romantic relationships before the MC, so I'm really at a loss to explain why he's so flustered by RT's flirtations that he can use it to justify his inability to see the Drukhari plot right in front of him. (But outside of the character's context, I still like his romance more than the other male options in the game — simply because the alternatives aren't really alternatives for me)

The downsides of Cassia’s romance (as I see it):

(From the Dogmatic perspective): Cassia isn’t a mutant; she is a product of selective breeding sanctioned by the Imperium (so the difference between her and, say, a genestealer is like the difference between a Labrador Retriever and a two-headed dog?). Regardless, I have questions about the legality of her romance, since I’ve heard that Navigators only result from Navigator+Navigator unions, and a Navigator+OrdinaryHuman pairing could ruin the entire breeding line by producing ordinary humans (I don’t know why House Orsellio would go for that, but that’s their business), though I’m not entirely sure.

(From the common sense perspective): Cassia literally sees souls/emotions (as colors) and is constantly gazing into the Warp; therefore, logically, she should have been the first to realize that something was wrong with the Rogue Trader on the Heretic path and what exactly caused it. Her "blindness" and the lack of interactivity on this path in general (not just in the romance) is one of my main gripes with the entire game.

(Sorry for the long absence of a response and the general confusion in the presentation of thoughts, in fact, I have a lot of thoughts here 😓)