A sex kick is an aerial which has a long-lasting hit box, with the initial hit dealin' higher damage 'n knockback when it's first thrown out and the long-lasting hitbox generally dealin' low damage and knockback. Sheik's N-air and B-air are sex kicks. Plenty'a characters have 'em. Link's, Fox's and Mario's N-airs are sex kicks as well, for example.
Sex kicks are ideal for gimps in Melee and P:M since their long-lasting hitbox lets ya throw 'em out with a little room for error in terms of the timing.