I'm not sure what the dress code is for the luncheon, so I'm going to wear just my undergarments since it is a safe choice.
That is not a safe choice at all. Why would you only wear undergarments?
Well, I'm used to be half naked around my own home.
Being half naked at your house is different than being half naked at a luncheon. Ask others what the dress code is, there is no way that will be acceptable.
WTF they are adults and they could handle it.
You may get arrested for showing up like that. Call someone to find out what you should wear. You need to obey the rules of the luncheon.