Newt has never been interested in sex. Companionship? Yes. Cuddling? Sure. Erections? Not so much. It's not until Graves mutters, "I'd take you like a erumpent in heat if it got you to shut up," under his breath does his cock sit up and take notice. Newt isn't even mad at Graves' interruption of his enthusiastic lecture of erumpent mating habits because he is suddenly achingly, blindingly hard at that thought of Graves mating with him. As if he too was just another beast in the suitcase.

Or Newt was never interested in sex until Graves accidentally stumbled upon his kink and now he's fixating on "mating" with Graves like a Niffler fixates on gold.