Am I the only person that really really wishes that the show hadn't gone there? Although it does provide an interesting back story, I'm not happy about the implications. Namely;
1) Are they seriously suggesting that Lin Bei Fong is bitter over her break up to the extent that it was the reason her interactions with Korra were strained prior to this episode? Really? You want to go the old woman scorned and isn't over it route?
2) I can buy that Tenzin and Lin grew apart. But even if you accept that, the implications of Tenzin choosing Pema over Lin are problematic. We know very little about Pema, except that, so far, she's had three airbending children and seems to spend her days at the Air Temple. It's probably the ideal situation for Tenzin, but far too neat. Lin clearly has her own motivations and goals in life; I'm not getting much for Pema outside of her 'true love' for Tenzin. It's...problematic, to say the least, that this is the situation which Tenzin prefers.
3) Tenzin left his girlfriend and friend (since childhood) for a fertile woman 16 years his junior. That...had to sting a little.
4) Between this and Pema's verison of the story (He was with the WRONG WOMAN and he was my TRUE LOVE), this is not making the Tenzin/Pema pairing particularly sympathetic. Audience is going to be more likely to 'side' with Lin Bei Fong, who was an original castmember's child, over Pema, who again, we don't know much about.