If you pick 'Yes, I want you', you'll have a sexual encounter with her. When you conclude by picking 'We'll talk again', she'll ask why Shepard is trying to get to know her and suggests that it's only so he can have sex with her. After you first recruit her in Dossier: The Convict, you can pick 'I want to talk' to get to know her. The very first 'romantic' opportunity you have is in your third conversation with her before getting her Loyalty Mission. Jack is also involved in a fight with Miranda, and your choice there can sever a Romance with her if you pick the wrong option. Notably however, you can have a sexual encounter with Jack before then, but it's considered a Fling and WILL lock you out of any further interactions with her on the Normandy.
