Quoting ~AJLR~:" Why do men find it absolutely nessisary to lie to their friends to make us look bad? Honestly can someone ... [snip!] ... halfway across the country right now or I'd probably be trying to kill him...or at the very least throwing things at him."
My husband doesn't do this. I do know that people who do this behind their spouses backs are generally not happy in the relationship and are "looking" for fulfillment elsewhere.
I would confront the issue. I wouldn't say "Why are you talking smurf about me?" but more "is there something lacking in our relationship that you feel we need to work on?" You actually bait him with this question to open up and talk rather than have a large fight about what he has or hasn't said.