Yeah, in the end if the writers have Superman having sex with a normal person it means his powers are controllable enough for it to work.

Pretty much. A Superman who can't control his powers well enough to have sex is a pretty terrible character, especially because if you apply that standard to anything other than said Superman's dick then he'd be a Superman who slaughters the world just by walking on it. So writers over the years have, for the most part, sensibly elected not to write about that character.