What's the proper response?

Men always talk about women and the dreaded "Does this make me look fat?" question and how to respond with the most positive results. Well, I have a question. Why do men always seem to ask "So where were you, with your other boyfriend?" Why do they ask that? Is it supposed to be comical? Is it suspicion hidden in jest? Is it insecurity? What is the best way to answer the stupid question?

I hate this question. I hate it with a passion. It irritates the hell out of me. I don't know if maybe I just read too much into it or what. I take it as a personal attack on my integrity and trust. Maybe I take it too personally. Every man I have had a serious relationship with ask this at some point. And of course, the truthful answer is I don't have another boyfriend. It is not even in my thoughts. I usually answer it with the rolling my eyes and a whatever or I start telling every little detail of my day so he doesn't think I'm lying.

Do men just need this reassurance or are they just making a joke? Is it just me that this bothers? How do other people respond to this question?