Yae was always a bit insecure, so it isn't entirely out of the blue that she would be worried about Chitose getting swept away by a guy with the way the sempai was talking in her club. It's still a bit odd, given that they both have clearly expressed their preference before, but Yae knows that Chitose has been at the all-girl's school the whole time, unlike her.
It's great to see Chitose hasn't gotten any more subtle! "I see. You think I'll start wanting a boy's **** in me?" LOL! That's what she's worried about, yeah...