I romanced Kaidan,but talked to everyone afterwards at the citadel. I rejected Diana, but Kaidan said we were gonna have another date but I didn't see him anywhere. So did I ruin the romance by talking to Liara or something? Edit: I locked into kaidan btw, does that still mean I can cheat on him by accident?
You can, as I recall, abort any romantic chain but it takes a conversation where you break up first (by insistence of the new romance option). Liara has an exception to this in the shadow broker dlc in the series but does not impact your choices in ME2.
Much of the dialog in 3 is meant to hint at what will happen after the game ends. I believe this dialog is part of that. It's an artistic tool to get you more heavily invested in the characters, as well as "feel" the ending more.