Whoevee explained this to you is a fucking retard.
You know how you can call someone pumpkin, or sweetie, or anything else of the sort. Kitten was in that category too. It's cute, it's diminutive, and it's playfully innocent.
Now kitten has a permanent unspoken "sex" in front of it. So it's now ALWAYS sex kitten.

Thanks for coming to my Ted Talk