If they leave fine, if they are a deranged sociopath and pick up and object on your porch and start bashing the window, then what? Or if it is a group of thugs, bent on home invasion, then what?

Then they're not knocking on the door to alert you to their presence, and patiently waiting on the doorstep, to give you time to go to whatever room you have your gun in and get it. Deranged sociopaths and "thugs" bent on home invasion are never so polite.

I would much prefer to have a gun and not need it than to need one and not have it.

Because you only imagine the situations in which having a gun works to your benefit, and never dwell on the scenarios where it makes things worse for you. We've had three people face charges because they had guns when they didn't need it, and used it to kill someone who knocked on the wrong door or turned into the wrong driveway or went to the wrong car. Those folks have had their lives destroyed, because they had a gun when they didn't need it. Those aren't the only examples, of course:

fox13news.com - Man wont face charges for university of tampa student shot killed after mistakenly getting into wrong car

Not everyone faces charges, but they'll still have years of wrongful death lawsuits. And of course, it can really mess you up when you gun someone down on your doorstep and later find out they weren't actually a threat to you. To say nothing of the situations where having a gun emboldens someone to escalate a property crime into a personal confrontation - vastly increasing the risk of personal injury to themselves from an encounter that wouldn't otherwise have reached that point. Given how rare it is that someone is breaking into a home for the express purpose of harming the occupants, rather than hoping no one's there, these other scenarios are far more likely for the vast majority of people.

And of course, that's only the danger to you, the gun owner. The other folks in these scenarios - who end up being killed or wounded for doing nothing wrong - would vastly prefer that the other person not have a gun when they didn't need it. This isn't like an umbrella, where if you have it and don't need it there's no risk or consequence to other innocent people.

Yeah - for most people, like the person in this anecdote, they'd be much better off not having a gun in the home.