How can I have meaningful dialogue?

You've come up against an all too common truth about the left. It doesn't matter what the subject, they are never able to articulate their opinion in a positive way because all of their positions carry a negative componant. Recreational drug use, healthcare reform, abortion, gay marriage - all of the usual liberal topics go against the grain for the majority of Americans. When you can't argue your position in a positive way, just fall back on the usual name calling and personal attacks, such as accusing the opposition of lying. Sorry, but "Liar, liar, pants on fire" just doesn't hack it in an intellectual discussion.