He did not, he said “lets be sex partners” never said lets be exclusive. Unless he confessed his love or is seriously pursuing him with the intention of something serious, he has every right to be with whoever. Though it should be openly discussed since u know STD’s and all that, but yaoi never really talks about that aspect. People just get annoyed because they want exclusivity from the beginning. I do not care since all he feels is obsession not love and until he makes it serious or they confirm feelings I don't consider it cheating (cheating is my turn off).
Good thing std's doesn't exist in yaoi lol
But they're only sex partners so far. Did he said he'd only see him? (I'm asking because I genuinely don't remember-)