Lack of Desire

Okay. So my bf and I have been together for 4.5 years. He just recently brought to my attention that he doesn’t want to have sex as much. He also decided to chime in that sometimes he feels it’s “just sex”, or he thinks I feel that way. Again, 4.5 years. We are in our early 20’s. I’m pretty disappointed considering I thought our sex life was actually better than it has been in the past. What could be going on? I don’t even know how to approach this situation.