What made him ask me this?

I have been dating this guy for a year now. while on one of our walks he busts out and ask me how many other guys am I seeing besides him. I was quite evasive purposely trying to avoid the question. Truth is, I am only seeing him but I know he sees others but I felt embarresed to say it was only him, knowing there should be others cause this does not seem to be going anywhere. Why do you think he would ask such a thing. curiousity killing me I asked him and he said it just popped in his head