"if he loved you and your daughter, surely he would plead guilty so she wouldn't have to testify"

It's possible that he doesn't consider himself guilty, and believes that standing up for the truth will ultimately be of more benefit to her than sparing her the injustice of someone else forcing her to do something against her will. If he is innocent, i.e. he did nothing that she had any objection to, then he has the right to say so, and you shouldn't blame the victim here, for the harm that other people do.

It's also possible he forced her, and his lawyer told him the best chance of getting away with it is to deny it. In that case, I hope he's found guilty and his lawyer falls down a flight of stairs and dies.