Shugo Chara would *NOT* be good if Amu had a penis. It'd be a shotacon+yaoi haven complete with a 10-year harem. I don't think you'll agree that it'll be good.
Like, male-Amu's three forms would be like...a marching band uniform, a French artist, and a butler outfit. I mean, just imagine...a male version of Hinamori Amu screaming "MAKE HONEY!!!!" in his green butler suit and flinging the sticky goo everywhere.
Oh god, and the transformation sequence and the "NEGATIVE HEART!! LOCK-ON!!" scenes would be...ahh, I'm going to fap now. brb.