It seemed to me that they weren't really attracted to each other, but to what the other represented. Jason Bateman's character wasn't ready to grow up, as you could see by the way he dressed and his clinging to all the old knick knacks and memories from his past. She kind of gave him a way to recapture a little of that youth for a while.
She, on the other hand, already had some of her childhood ripped away when she became pregnant. Hanging out with him allowed her to test the waters a little and see what she thought being a grown up is like.