When something like that, people go into the fight or fright mode. In this instant the mc when into fright, stuttering, talking nonsense, making up lies, to try and convince the ml that everything was fine but ofc he didn't buy it, and just went even more pissed when the mc lied.
The main reason why he didn't told the truth that maybe to not hurt the ml or to not ruin the relationship. Idk.

In bdsm theres always a save word. And this occasion its 'I love you' and the mc said it when he had a chastity belt on.
And boundaries, I can see the ml having a boundary as "always answer the phone when I call or text." Or it can also be "dont stay out drinking until late" cuz so far he has gotten mad when the mc doesn't follow them.
But for the Mc boundaries, its hard to tell cuz so far nothing has tell me "hey. This might be one of the Mc boundaries that the ml is breaking"
But there's definitely no communication. And the ml doesn't really know what happened at the bar. Only he knows its what someone told him which is "i saw the mc leaving the bar with some guy" and ofc when u hear that, ur mind goes right away into "he's cheating" and the anger becomes so huge that theres no place for reasoning or listening. (Believe me, it happen to me. Not the sa' part tho)
And i feel the women we saw at chap 17 its the same women that help ml with the whole bdsm thing, so she might told him sum like "hey is this ur bottom? Cuz i saw him leave with some guy at the bar".
Okay that's all. Again its what I think. Agree or disagree its totally fine.