@breizh,
@gigabarto, and
@Zirvano: What are you, daft? They reacted more to each other's opposite-sex forms because:
[ol]
[*] They're totally new to the sensations of physical arousal of the opposite sex.
[*] They didn't expect each other's opposite sex form to be so attractive.
[*] Specifically in the case of Leo's reaction to female Fooly, the traits that make Fooly physically "unmanly" (slender build, soft face, etc) are more or less things that make girls cute; that being said, the big breasts are obviously more for comedy than having a rational explanation, unless it turns out that Fooly inherited genes for big breasts and they were simply dormant due to him having XY sex chromosomes instead of XX.
[*] It's logical that Leo has even more trouble dealing with the genderbending (whether it's Fooly or her that undergoes it) because she spent her entire life shutting down her libido to focus on being a warrior, thus making her ill-prepared to deal with it when Fooly managed to reignite it into a rapidly growing raging inferno (first by charming her with his sincerity, loyalty and overall good nature, and then by turning into a surprisingly cute girl with a voluptuous figure).
[/ol]