@Miyuki Besides the people who know Rimuru is a slime, he doesn't talk girlishly and of course has gone to the cabaret club several times, so maybe they suspect he's "male"? It's a rather muddled subject, because for as much as he's corrected people every once in a blue moon, he also seems to not be bothered by the girls doting on him and dressing him up in his Shizu form. So on the whole, much like a slime itself, I think Rimuru kinda averages out to somewhat... androgynous?