"soñar con" means "to dream of" or "to dream about" "видеть во сне". It does NOT mean to sleep with. (спать с) Therefore, your Spanish sentence means "I dream about my girlfriend." "Я вижу свою девушку во сне."

"sueño con" means "dream with", actually. You'd be better off using "duermo". "Yo duermo con mi novia."

Thank you, Andres :)

I dream about my girlfriend is NOT the same as I sleep with my girlfriend Yo sueño con mi novia Yo duermo con mi novia

Yes, it is ok.