That is a correct assumption by you in my opinion. Still one thing that I (and a couple other players) are a bit baffled is that Mina NOW starts this. Ian would have been more than happy to help Mina scratch some of her kinkier itches, it might have even stabilised their relationship quite a bit.
But e.g. recall the "Truth or Dare" game where Mina played the role of quasi-innocent. It does not fit her "bucket list". And more, even when Mina and Ian´s relationship was really well, she did not talk with him about "experimenting". And we also know how Killian is, he would not have been shocked, but happy to find a girlfriend as kink-happy as himself. I truly doubt Mina could have mis interpreted Ian´s openness in the bedroom. Just look at the conversation they had during "Truth and Dare" about tagteaming.
I think it comes down to trust and I think that even from the moment you first meet Mina in the club with Killian the game makes it very clear that she already has had issues with him and doesn't trust him completely, despite her slightly naive attitude towards their relationship. She wants to explore these things but didn't trust Killian completely to do so, and let's face it, he makes it clear that he never really treated her particularly well from the start. I mean, he more or less outright admits that's how he has acted in all relationships.
So now Mina ends her relationship with him, finds some guy she thinks is actually trust-worthy and also is undergoing a bit of a moment of self-realisation. It's just all sort of adding up. If Killian had just earned her trust a bit more then possibly she might have asked him to help her explore (although you could also suggest that maybe she has some hang-ups over him being her first "boyfriend" and being too struck by the novelty of that all to make the move there)... Overall, I feel like the progress of her character makes good sense to me.
Plus, you have to account for porn game logic and the protagonist has a magic cock so...
On a totally different note, I'm playing through 4.2 now and it's finally fucking happened. The moment where you have to choose between some of the girls. I knew it was coming and have currently decided to go with the "lie your pants off and shag everyone" route, but godamn it made me feel like an absolute asshole for doing so.
As always, very few games absolutely nail the porn as well as Pale Carnations while simultaneously also making me so invested in the characters. I said almost exactly this at the last update, but HOW DARE YOU Mutt and Jeff make me feel so bad for just wanting the porn... When that choice inevitably comes back to bite me I'm gonna be bloody furious...