I think it just clicked with me what the difference in 'vibes' comes from here.
I do not engage with any sexual content involving Eubicha, so when I introduce her to Kaly there is no sexual ties between me and Eubicha. For a character/player who is more of a dominant or switch, I can understand that the vibes may be different, as you are then actively offering a submissive who you have sexual relations with up for Kaly to dom herself. For me and my PC, since we've never fucked, she is just a friend I am introducing to Kalysea to have a threesome with.
From my perspective (subby fem) it feels like a waste of the characters to not give them scenes with Kaly, since otherwise I'd basically never engage with them sexually.
I hope that makes more sense now.
Edit: Except Eubicha's bad end, that's pretty hot, but not canon to Eubicha x PC of course.