Is being cheated on a red flag for females?

Obviously depends on the situation hence the stories of chicks going back to that 'douchebag' again and again.

If the guy's value is high enough, then no it won't be a red flag, it's probably an extreme DHV + Dread that creates drama she can't help sink her teeth into before having pornstar make-up sex afterwards.

On the other hand she could a be cool, switched on chick, in which case she probably wouldn't put up with it because although it may still trigger feelings of jealousy and even visceral arousal, she may have strict personal boundaries against that kind of easy-going relationship - and may want a more robust structure for her stage in life. Maybe she's approaching the wall and wants a reliable ATM/Alpha seed/Source of Validation.

It really depends on the specifics of the people involved in any give situation.