Thailand is one of those places where just about anything is available at a price.
I was never offered bareback sex over here and never asked for it ( I am not that stupid).
but I am sure if you offered the girls extra many would do it.
The girls who work in shops probably would not as the shop might have a rule about it, but some of the the freelancers and bar girls would do just about anything if you offered them enough.
You would have to be crazy to do it though.