Galahad isn't exactly fused to her voluntarily; He was summoned by Chaldea and would have just given them the finger and fucked off, but if he had done so Mash would have died right then and there. So he fused with her as a Demiservant to keep her alive, but refused (initially) to allow her access to any of his abilities. Once the Singularities are resolved, he cuts off access to them again. Galahad is An Asshole and Percival is the superior Grail Knight anyway🖕
As an aside, some of her vitriol towards Lancelot's womanizing ways comes from their fusion, but most of it is genuine.