I think one big reason people are ignoring (and ignored in the first chapter this happened) for Leo's more extreme reactions to Fem!Fooly is that Fooly goes from being an average, albeit adorkably cute guy, to an incredibly, absurdly attractive and *developed* female. So whether you're under the assumption that she's bi, or "Fooly-sexual" it makes sense that Fooly's female body gets a bigger reaction out of her.