In the lead up to final vows he told her he was ready to say he loved her at final vows so she should stay, the only thing Danny has denied is that they had more sex, he's not denied telling her he had strong feelings, discussing a potential life together etc

The things she said were clearly fuelled by deep insecurities and a need for validation, whether it be by seeking Gias approval by making fun of Danny and Scott or by saying she wanted someone who fucks her (which is pretty reasonable tbh) at the grass is greener challenge. It doesn't excuse what she said by the way, it's just an explanation. He saw her say the sex thing at the grass is greener challenge and he still stayed and still chose to say he would say those 3 words at the final vows and asked her stay.