I asked my recruiter if I could have my Korean girlfriend come over to my accommodation on the date I arrive. But my recruiter said it is not a good idea. Why is it not a good idea? 

I totally understand that you must feel upset about the recruiter’s answer. I think your recruiter is concerned about your first impression to employer. 

When you arrive to Korea, you will be driven to the hagwon and the hagwon owner or supervisor will welcome you. They are the people who will spend the most time with you at work and they are people who will take care of you while you are in Korea. 

Fist impressions are important. First impression doesn’t mean your appearance.  It means your attitude and mind set.  Schools are expecting a hard worker and someone who can put their heart in their work. Your recruiter’s reaction might be caused from the concern of that your employer might think your intention to come to Korea is distracted or more focused on “non-teaching” 
 

Meeting your girlfriend on the first night is not a problem, but speaking as recruiter it is would be wiser to meet her on the next day or later after getting training, adjusting new working environment, learning teaching skills and learning textbooks. 

Your recruiter has your best interest at heart and is worth to listening to, but you are the person to make the decision.