A toggle system sounds good actually. (Interested / not interested).
I recall in the Mass Effect series wanting to get to know the characters better and so I talked with them. I remember having to reload several times because I was suddenly forced into a romance unawares.
Ashley in ME1 was coming on full speed ahead for one, and there were others. you could easily cut yourself off from dialogs with other NPCs.
Dragon Age was another game I remember getting into some "uncomfortable" dialogs with players as well..."Hey watch where your pointing that sword!".
