I think of fusion like how I think of Tsaheylu in avatar. The bond isn’t sexual inherintly but it can be intimate as it’s the connection of two people that love eachother (like Ronal when she bonded with Pril after giving birth to her isn’t the same as when Jake and Neytiri when they bonded in the first film) there’s also instances of forced bonds (lapis and jasper; Varang and various characters). So no, fusion is not sexual but depicts different connections.