1: All the girls act yuri for service-service, but they don't get visibly flustered and uncomfortable. Calli gets flustered and screams really loud because it's too real for her.
2: She hangs out with guys like Connor IRL not because they're fucking but exactly because he's a guy and so she can be in an environment where her heart is constantly pounding and her pussy isn't throbbing.
3: She claims to like small guys but it's all a ruse. It's like a closeted gay guy claiming to like muscle girls. Manlets are basically pseudo-women, so they provide just enough cover while allowing her to sound invested.
4: Here's the clincher that proves it for good: when she recounted the story of her lost love, she A: makes sure to not reveal the sex of the person, and B: says they had to figure something out with themselves. This makes it very clear that she's saying that the other GIRL couldn't decide if she was a lesbian or not, and sure enough she kisses a guy at a party and Calli cries. That detail of the story doesn't make sense otherwise.