Bi dudes who flirt or hit on other guy's while in relationships.

Just something I don't quite understand, I know not all folk do this but it's kinda weird to me, & to be honest appears like obvious cheating in a way. Not that any encounter I had went beyond a few texts recently on social media. Why do people give the impression that they're Bi & available but are involved with the opposite sex, I know this isn't really a big thing for some, but it's just a little pet peeve I kinda have. People make it seem like it's no big deal, I kinda feel for people out there who seriously wanna relationship & have to deal with this stuff on a deeply personal level.