Selfie with Shimakaze !
Because the design is factually more interesting if it's a boy. As a girl it's just another slutty fanservice character that has been seen a million times, but slutty boys are really uncommon unless you specifically look for slutty boy content regularly. Also in terms of design, a dick creates a focal point where previously there is none. Usually a character will have a design element that draws your eye, a hair style, a necklace, huge tits, arm band, fucking something. Shimakaze is just really generic looking and has nothing that pops out (lol). Adding a bulge or a dick literally adds a design element to the character and makes the design feel more complete.
It's really fucking stupid and literally is exactly that trope image of
>wow what a shitty design hurrr
>BEST CHARACTER EVER MY WAIFU OMG
But that is actually how it is.
I want to bury my face into her butt