HHHHNG I CAN'T THINK OF ANYTHING WITTY!
UHMMMM..... well let's be real here then. Currently I imagine any male reproductive organs being internal.
Knights can only reproduce with other knights or other possible exoskeletal races, depends. Because makin' babies with a non-exoskeleton race would not go well.... like, at all.

Aw jeez. Welp, it ain't pretty, but......
Skeletons are way up there, human ones to be specific. I typically draw them to be very cutesy.
Armor suits~. I love'em. So much so that when a character is wearing one, I typically love them for the way their armor looks, and not so much when they take it off.
Almost any sort of monster/demon-looking character.
Some Pokemons.
Being dominated. I'm not exactly in to being tied up and whatnot. I just like bigger guys telling me what to do... passionately. >n>
That's about it really. That sort of hurt... Ugh~