I would have liked a romance arc with light sided Jaesa, but it wasn't in the game so I followed it with Vette. But, once the conversation arcs are completed, other than some mail she sent, there's really nothing left. You can't just go to her and flirt/kiss/fade to black with her like you could with Morrigan in Dragon's Age, for example. You just finish the arc and then that's it.
As far as dark sided Jaesa becoming promiscuous, it's because Sith follow their passions.
And light side Jaesa isn't really bloodthirsty. She says things such as "their pain is exquisite" because she's playing the part of a dark Sith apprentice.
She does still consider herself a Jedi, at least for a time. It's not until the last conversation that you have a chance to mention children to her. If you're married to Vette then you can mention that you encourage her to get married to and that you'll raise and train her children as if they were your own.
If I were to play my character again (and I just might, I've really enjoyed the Sith Warrior story line) I'd probably bypass the Vette romance option. Once I got Jaesa I just stopped using Vette because Jaesa compliments my immortal juggernaut so well. And it just feels funny that my character has his "wife" back on the ship while he runs around with another woman. Plus, Jaesa doesn't dislike when I'm respectful to people in Imperial authority positions like Vette does. So Jaesa just fits the way I play my character better.
Plus, lightsaber sounds are cooler than blaster sounds.