Is she dumb? She is straight. Of course, they will fuck if you are not there, duh!
After all these years, she was talking about boyfriend A, B, C and how she lost her virginity with a dude, but you have to see it with your own eyes to finally believe it that yeah, she's straight, and she's not going to change just for your pretty eyes.
I truly hope she will finally turn the page! Even if it's not with her senpai, but just move on!
And what happened to Eri is tragic and awful. If she manages to find happiness and love in the hands of Kyo, go for it!
Even thought the way Kyo describes his motivation to date her are....meh. Like "All the dudes you dated before were awful to you, but I will be different, I will be the chosen one, the prince in shaming armor, the one who will give you eternal love!".
Dude, chill, you don't even know what she likes. You need to ask Fuyuki for advice.