To answer this question we must first recognize what a "sex kick" is. What would a Sex Kick be? Your definition is correct, a sex kick is indeed a move that first has a strong hitbox and then has a weaker lingering hitbox. However, as the word says, a sex kick also happens to be a kick.
King Dedede happens to already have an aerial "kick" what exactly the same properties. Wanna make a guess? Correct, it's Back Air! King Dedede already has a Sex Kick. A character having two sex kicks would be confusing, because that would beat the whole use of the term. You couldn't say "Dedede's sex kick" if both N-air and B-air would be one.
Another problem that arises is that, despite having the same properties, it doesn't have the same use. Usually weak parts of sex kicks are used to just poke someone who is in the air (recovering). This is completely pointless with N-air, because it pops someone up.
Also, it's a bellyflop.