I am curious about laws and practices regarding marriage and other sexual relationships.

Is it currently legal in Thailand for someone to have more than one spouse?

If not has it ever been legal?

How common is it for several unmarried people to live together as a polyamorous group?

Is it now or has it ever been customary for people to openly have extramarital affairs and/or relationships or do they lie, deceive and cheat as is common in the West, or is it really not an issue for some reason?

Please elaborate as much as possible on these questions and related topics.