@donm527 He already asked her on a date, just finished the date, and asked her, "Would you like to come back to my place?"
So she agreed to the date, had dinner and wherever else he had taken her, and then when asked to go back to his place, "Sorry I have a boyfriend."
If she "gets that all the time", then she should've just said she had a boyfriend when he asked her on a date! Fuck, the confession is basically when he asked her out!