Well, they are not teenagers, they're actually adults with jobs and stuff. Mako is a cop on bodyguard duty, Asami runs a company, and Bolin joined what he thought was the peace corps. The main cast are adults, and adults make mistakes all the time, if the divorce rate in my country is anything to go by. Bolin is just
too trusting and forgiving, and Opal is not trusting and forgiving enough. Maybe the creators thought this would be a good way to bring pain or complications to what some of you think is an unimportant comic relief character. (as if the comic relief was unimportant!)
Bolin is a sweet guy, but let's face it, he is not a clever man.