You see, for me the scene fits. This is the woman who told us to "go with the flow", because that's her philosophy. So, yes, she goes with the flow. And yes, she knows how the MC's ex betrayed him, that's why she's honest about it. Yes, she wants a relationship with the MC, but people can't change on the drop of a hat. I'm not saying she'll go cheating on the mc and fuck as many women as the MC, but she's not going to stop flirting because that's been her way all these past years. This was her letting the MC know that's her life. You can't expect a woman who reads all the time to stop from picking up a book in a heartbeat just because you only like movies.
I can see your points but don't necessarily agree. To me it shows a lack of empathy as a person to know the MC's past as well as she does and immediately push his limits. She doesn't know that he is considering schtupping every student and teacher at his school (in my man-bear-pig-whore playthrough), no dialogue has been written that is of him talking about his recent or future conquests. She knows about him and Mara and him and her that is all.MC and Kate have known each other since childhood, so while this may be their first official date, they know each other very well. She is expressing her sexual impulsiveness and lack of limitations on how that impulse is satisfied, and IMO is the sexiest thing I believe I've seen yet in any game.
She would've crawled under that table even if her brother, their parents, their grandparents, etc. would have been sitting there. This impulsiveness is influencing MC. He's breaking down those walls of what he would consider social norms. He's putting his own livelihood at risk to finally fulfill his repressed sexual desires while he still has the chance to do so.
And to the point about her knowing him since childhood it would mean that she should know that he was a person who valued monogamy and faithfulness but felt he wasted his years with Elaine. Mara could have been an aberration or true love for all Kate knows. Yet she is immediately eye banging the waitress and telling the MC about how much she loves being fucked by women. Kate even chose the restaurant, she's familiar with it, so she could have already had that waitress prior to that dinner if she had dined there before.
Overall, I just felt that the conversation was not that of a LI but rather of a fuck buddy. It seems all she really wants is to fuck.