I think it was less of him listing it as an actual turn off and more of him just trying to get the MC to admit that he's okay with dating guys.
No, then it'd be a whole different question and it'd be worded differently, not added to the "is it because" list he's throwing at him. Since ML knows MC dates men he could've said something along the lines of "I know it's not because I'm a guy.", which would promptly reveal that he knows about it. Either way the MC's response is what doesn't make sense here; the natural progression here should be MC either admitting that it's not because of that because he does indeed date men, or not admitting it by wiggling his way out of answering completely. So, either the question should've been worded differently or the answer, for the word exchange to make any sense.
The translation in chapter 11 is a bit weird or maybe worded weirdly; ML is listing possible turn offs as to why MC would reject him and he says "Is it because I'm a guy", so why is MC's reaction to that "How did you know I dated guys?". Makes no sense. If ML knew MC dated men then why would be list it as a turn off?