Now you may be saying to yourself "but LemonLime, Visas already had control over the force" which she did but she was also a Sith. The feelings that developed in the time before the master/apprentice relationship begins still persist, this is why you end up with this pseudo romance. Once this relationship changes a true romantic relationship carries a certain stigma. Once you begin their training the relationship between the characters change from peer/peer to master/apprentice.
So the romantic feelings between the two characters begin to develop while the two are still travelling together, still relative peers. With the exception of Visas these are all characters who are force sensitive and you can train in the ways of the force but you travel with them and begin building relationships with them before you begin training them. Ok so the character's you can enter a pseudo romantic relationship with are, if I am not mistaken, Atton, Handmaiden, Visas, and the Disciple. The reason being that there really wasn't one but you could tell there was a sexual tension between the characters but the reasoning behind it with the writing is whats truly exceptional. In my opinion KOTOR 2 has the best romance subplot in any game of recent memory.