There is no way to explain why it is bad without spoiling it, obviously.
To use a metafor:
You go on a first date with a girl, there is certain chemistry and you two decide to meet up for a second date. This second date is amazing, it's far better than the first and you both have a great time.
The third date is also pretty amazing, you start to realise this girl could be a keeper. The fourth date goes a little rough, perhaps she's having a bad day. Or you, who knows.
But still, you meet up again because the first three dates were just great time. This fifth date she asks you to come home with her. You've been looking forward to this moment for a long time, all the previous dates have been leading up to this moment. She takes you to her bedroom, sits you down on the bed, and just when you think she is about to undress she says "I really like you as a friend".