Maybe Mahiru wasn't gay, but Sayo just realized she was.
Wut? It's very clear Mahiru wanted to get together because she figured out her sexuality enough to know she likes Sayo. She, out of the blue, asked Sayo about if she has a boyfriend as one of her first orders of business, and then replied coyly while twirling her hair that she was cute.
Mahiru is very much the confirmed gay here.