Because when you open the door you expose yourself to the public. I don't know about Thailand really. In the US you can be naked in your own home or garden of there if you could not be reasonably seen from a public place outside. *Curtains, fences, hedges.
For answering a door naked however, various states prosecutor could suggest
Incedent exposure - Do you have a ”lewd intent”? Was you behavior likely to cause offense?
A civil claim could be brought for sexual harrament. ie Clinton supposedly exposing himself to a woman in a hotel suite.
Is there a school bus stop outside your home?If it is a child involved some states like Texas could demand a custodial ”life entence ” of prison.
A few years ago a man in US was making a long phone call while free balling in front of the tinted windows of a Westin DEN airport hotel. Based only on outrageous claims of passersbys at the terminal police barged in and arrested him. Eventually he got large settlement as the Police violated various procedures, and his rights. The man had no idea he could be seen from outside.
https://www.nbcnews.com/news/us-news/pilot-arrested-being-naked-his-hotel-room-window-paid-300-n1080756