Why do married men flirt?

Why do married men flirt, make it known they want to get in your bed..and then get all defensive and start talking about their marriage and their wife when you engage them in this flirtation? I can't call it. I am in the midst of enjoying a little flirting with this guy I work with and he IS interested in me, that is very clear, but talks about his wife constantly. I am getting really annoyed with the mixed messages. I just don't get guys.