It's typically understood as when a person is cheated on.
In a game like BaDIK, it would be like if one of the main LIs (Jill, Bella, Maya, Josy or Sage) cheated on the main dude after they were dating the main dude.
Some people get off on the idea of being cheated on.
Of course if it's happening in reverse (like the mc fucking Jade, who is a married woman), none of us give a shit, except for her husband, Stephen, but then again, he looked like he was getting off on it when he was watching that video. Now hopefully that shit raises its head next episode!
But yeah, some games like to put the hero in an absolutely devastating situation where the love of his life went off and fucked some other dude. Some players have a total meltdown when it happens, some love the drama, and some couldn't give a fuck because, pixels...
In the end, it's a highly polarising plot device, that has its niche, but is typically avoided like the plague in many games. BaDIK seems to be one of those games that avoids it due to the total lack of sexual history with most characters even though they seem to be highly sexual beings.
Ultimately it shouldn't even be discussed in this thread because it has fuck all to do with this game, but people love arguying about it for some reason.
Fuck, I just got roped into the discussion didn't I...