well.. a lot to say here: what are you looking for?
As a couple, .. are you looking for swinger couples? (give a try to SDC.com. but I do not recommend to go to a bar and "find a couple" as most likely she will be a hooker.
if you are looking for a bi lady, or a ladyboy (kathoey) you will find plenty, but it is prostitution.
If you are looking for a man for threesome probably you will find some in night bars/disco. Also i do not recommend you that.
So, my piece of advice is enjoy the best of thailand: beach, food, natural parks, and temples. Enjoy the kindness of the people. and climate.
It is a fantastic country.
The prostitution hotspot are concentrated in 3 cities: Pattaya (a shithole), bangkok (Nana plaza and Soi cowboy) and Phuket (in Patong beach).
Thai society is very very convervative and outside of those hotspots you can find prostitutes but it is not as easy as you. might think.. and it is like in any european country.
I lived and worked in Thailand and I love the country and the real people.