Was I justified on not wanting to grind with my prom date?

I think that you were right and you shouldn't feel bad. If you had let him grind with you, then you should feel bad. He had his chance and he didn't take it, so he should expect that you would find someone else. Just apologize to him if it comes up again about not telling him that you had a boyfriend. He probably just mostly felt stupid for not realizing that you were uncomfortable and taken.