Being a Thai Monk is a societal requirement for Thai men, but it's not a government mandated practise.
When I was there we'd often meet the monks walking down the street, after the Bangkok bars closed down, lol. I was with my Thai friend, and we would always throw a few Baht their way. My friend did his 6 month Monk stint when he was younger, and had some great stories to tell me about some of their adventures (mis, most of the time =)