If she would have made the decision for another kind of boyfriend doesn't really enter into it because we will never know. What we do know that faced with similar scenarios, she made the decision for Zuko her boyfriend that she didn't - couldn't make for TomTom her brother.

It doesn't really say anything positive for Mai as a character that she allows herself to start caring for her boyfriend in a way that she couldn't allow herself to care for her brother. It only explains her character, not excuse it.