Well you can't romance Jacob in ME3 either^^.
The 'locking in' with Kaidan is only available if he survives the Cerberus Coup, letting him joining the Normandy and talking to him on the Presidium Commons afterwards (plus being nice to him during your visits in the hospital).
Might even be, that the Geth Dreadnaught needs to be dealt with before he is available on the Presidium too.
All in all, before you even meet him on the Normandy Docking Bay, you'll bid your farewell to Thane in the Hospital and like Liara after LotSB you are free to go to whoever is available.
