Someone once told me that the Ilia probe need not have even been female, for Decker to have been motivated to join with V'ger. Saying that he did so because it was quite historic and once-in-a-lifetime ... and all of that stuff. Truth-be-told, it's not entirely clear, in this movie, what Decker's real motivations are.
There was a deleted subplot involving Decker's interest in spritual unity and transcendence, which was part of what had motivated his interest in Ilia and the telepathic unity of Deltan lovemaking. V'Ger gave him the opportunity to experience that kind of transcendence. Unfortunately, that was all lost.
Then again, if it had been left in, we might feel it was dated, since it does kind of reflect '70s culture and the mystical/transcendental movements that were trendy at the time.