User blog comment:AdmMiller/Who's pschyed up for ME3? (Romance options only)/@comment-2257260-20100816031626

...Liara won't fight anyone for Shepard. She's completely fine with an open relationship.

If you begin romancing both Kaidan and Liara in ME, and don't turn down either of them, they will both confront you, asking which one you want. It is possible to ask "Can't I just have you both?". At this point Liara will say that she is completely fine with this, but Kaidan will turn you down.