I think I figured out why I'm obsessed with having sex with Finana. It's not a matter of love, romance, or lust. It's a matter of trust and communication.
Finana is clearly a very kinky and openly sexual person, but she's not really a slut of you ask me. I highly doubt she sleeps around much and while I wouldn't be surprised if she's fucked guys she wasn't dating before I can only imagine she did so because she trusted them enough.
So right away having sex with Finana also implicates having her trust and vice versa. Trust meaning that you know she won't make you uncomfortable or that she knows you well enough for this kind of experience to be easy to talk about. You also would be close enough to her that she feels comfortable letting you see her exposed and touching her in such a state.
Communication should be fairly obvious. She's pretty open with chat and her gen mates about her kinks and interests, so if you were to have sex with her it'd be really easy to talk about what you both want to do or see. If you're new or awkward she probably wouldn't judge you too harshly and would just sweetly guide you on how to improve as a sexual partner.
When you combine these two aspects, as well as her incredibly cute design, you get someone who I have to assume would make for an amazing sexual partner. One that, realistically, would actually be a lot of fun to fuck even if you're very inexperienced or uncomfortable with sex. Because of that, I genuinely love Finana Ryugu and 100% believe in her seisoness.