How we cite our quotes: All quotations are from Star Trek: The Motion Picture.
Quote #1
DECKER: I'm sorry.
ILIA: That you left Delta Four? Or that you didn't even say goodbye?
DECKER: If I had seen you again, would you have been able to say it?
Wowza. If their longing glances during their first meeting didn't tip you off, then this should make it clear that Ilia and Decker used to be an item. It's also clear that both of them still have some residual feelings.
Quote #2
[Ilia is seemingly disintegrated by V'Ger.]
DECKER: This is how I define unwarranted!
Decker is a lot more of a nebbish than the always-courageous Kirk, so Ilia's death is his worst fear realized. These two lovers were reunited just to be torn apart again.
Quote #3
SPOCK: Suppose that beneath its programming, the real Ilia's memory patterns are duplicated with equal precision.
[...]
KIRK: Ilia's memory, her feelings of loyalty, obedience, friendship might all be there.
SPOCK: You did have a relationship with Lieutenant Ilia, Commander.
How would you react if your dead ex-girlfriend came back to life as a super-advanced android? We'd freak out. (Or maybe make out with it—we've always wanted to kiss a robot.) Personal fantasies aside, this is going to be an emotionally draining experience for Decker. He's still grappling with Ilia's death and now he's expected to exhume the grave, metaphorically speaking.