Well I guess I'm in the minority that sees what Christine did as being totally plausible.
But that i guess depends on character assumptions.
Christine to me always seems to be a flaky, naive person, even at the end of the first one. After all she believed the Phantom was her father's ghost, or at least a spirit sent by him.
And on some level she does love the Phantom or at least is infatuated by him.
What happens does not happen the next night, it could even be 6 months later, the time frame isn't stated. Just that it happens the night before the wedding, who knows what happened in that time between herself and Raoul. It is well known in a lot of courtship situations that people put on a 'face' in the start of a relationship and the real person comes out later. Who knows if Raoul started to show his later 'attributes' shortly after the whole Phantom incident.
Anyways, it is not totally far fetched that she went to see him one last time, not for love, not for sex, just for pity even, to be sure that things are over, and things happen. It happens all the time, people get caught up in emotions and things happen, just look at Jerry Springer show, lol.
http://www.flickr.com/photos/27199361@N08/ Phantom at the Royal Empire Theatre