Technically it wasn' t a Democratic debate, it's just that all of the Republicans are too homophobic, and turned it down. Yes, Ron Paul hates the gays.
It's nothing new that Ron Paul "hates teh gays," or whatever, but it's simply inaccurate to classify all the Republicans running as "gay haters." Giuliani has a record on gay rights that's better than most Democrats in this country—it just makes no political sense for him to go to a LOGO debate. It'd kill him amongst the GOP electorate who are already wondering if he's too socially liberal.