I believed he suspected Ulra was a man. IF you looked at the way he was concentrating on him while he dance. I remember reading a manga here where the seme was attracted to a female he did not know why since he has always like males, in the end he found out that she was a he. Azim knew it was a guy.
Because he knew he wasn't. Would your first reaction to someone trying to kill you be to check their sex? He knew from the start.
And the one you're referring to does a collaboration with Yukine Honami or Satoru Kannagi I believe. That author did a much better story there than this author did with the one you're talking about here. Ugh, this one was AWFUL.
But I also have a question, I don't think he 'suspected' Ulra was a man. After all, in the second chapter it's explained that the father was trying to 'protect' his son and so Ulra shouldn't be complaining about anything his father did. Ugh, gaslighting and victim blaming to the MAX. Just a bloody AWFUL story.
I may just be being nitpicky here, but, in the first story, if he has no interest in women, why'd he take Ulra back with him in the first place, when he thought he was female?