It's hard for parents to watch their children make wrong decisions, and of course, we need to do what we can to steer them in the right direction. But as they grow older we have less and less control over them. That's why there may be very little you can do to persuade your daughter to change her mind about this.
But there is one thing you can do now that could be very important in the future -- and that's to let your daughter know that although you disagree with her decision, you still love her, and you always will. Relationships like this often don't last, because there's no real commitment behind them. And if this relationship ends, your daughter could be very hurt -- and she'll need your love and support.
Do you remember Jesus' parable of the Prodigal Son? (You can read it in Luke 15:11-32.) At first, everything seemed to be going his way; he probably laughed when he recalled his father's warnings. But then everything changed, and he was left with nothing. Undoubtedly, his father had assured him of his love -- and with this memory in his mind he decided to return home. When he did, the father joyously welcomed him. "'This son of mine was dead and is alive again....' So they began to celebrate" (Luke 15:24).
Pray for your daughter; God loves her, and He can bring her to her senses. In the meantime, let her know you still love her, even if you disagree with the decision she's made.