If your friend never introduces you to her boyfriend after 3 years what would you think?

I have a friend for 3 years now and she has never introduced me to her boyfriend. I think this is really weird but I just ignore it. I have even invited her to social events where she is welcome to bring him but she never does. She showed me a picture of him once after I asked her but that is it. They live together too and I know he is real because he has pictures of her on his facebook but she has none of him on hers. I just think it is weird and creepy. I feel like maybe there is something about him that she is ashamed of. How would you act in this kind of situation? ?