In the case of Ajantis, [spoiler]the attraction is over shared values and personality traits, and it takes a while to develop. It's very natural. Quite different, really. The niece doesn't just say "Oh, Ajantis, you are so handsome, I must have you now!" And the attraction is mutual, which is also appealing. *And* it doesn't happen at all if Ajantis and the PC are in an active relationship, so it isn't like it's nothing but a cheap ploy to draw attention to an attractive NPC.[/spoiler]
With non-joinables displaying instant attraction/throwing themselves at a party-joinable NPC, it's got depressing Sue overtones. "Oh, look, even strangers in the street can't help but notice how wonderful I am! Don't you think I'm wonderful, too? After all, look what I'm passing up, all for you!"
I don't mind joinables hitting on other joinables, either. Korgan makes suggestive comments toward everything female, and that's fine. Kivan is deferential toward Aerie, and that's OK, too.
Nor do I mind NPCs, joinable or otherwise, that are designed to flirt with others, any others, not just specific NPCs.
Example of something I would find really obnoxious and unacceptable:
Acolyte Lara: ~Gavin, would you stay and talk a while... alone? It's nice to meet someone with such a powerful... faith.~
Something that I would find acceptable, but badly written:
Acolyte Lara: ~OR(4) IsGabber("B!GAVIN") IsGabber("Anomen") IsGabber("Cernd") IsGabber("Keldorn") ~ ~<GABBER>, would you stay and talk a while... alone? It's nice to meet someone with such a powerful... faith.~
Acolyte Lara: ~Class(GABBER,CLERIC_ALL) Gender(GABBER,MALE)~ ~<GABBER>, would you stay and talk a while... alone? It's nice to meet someone with such a powerful... faith.~
Everyone has different deal-breakers, but that's a big one, for me.