----- Not an ESL teacher -----
Then, by reading your post, I had to stop to think about it.
Usually yes, but not necessarily. It is somewhat of an affective way of expressing a relationship (although someone may object that it also expresses a "possessive" feeling implying some prejudice).
You could also use "She is my girl" to refer to your fianceť, wife, or even your daughter.
Student or Learner