Fully gay and fully straight are cultural categories that we use to navigate in life. This is especially true for younger guys, even though they on the surface tend to be very firm in claiming to be straight unless they are gay. Luckily, in Thaliand and other SE Asian countries, the gay/straight categories seem to be tight to the public image and not so much what you actually do in bed, e.g. the example of seemingly straight Thai boys with Thai girl friends accepting varoius offers from farangs to join them in bed for a small amount of pocket money. I personally believe that the money plays an important role in the this - the money can (totally unconsciously!) be a good psychological excuse for the Thais boy to join the farang in bed. It might not lead to a glorious money boy career for the boy, but it can serve as an opportunity to explore your sexuality.
I consider myself gay, but I know that it's not entirely true if I claim that I have only thought of boys and guys since my young teenage years, but it has MAINLY been guys and boys for sure, and over time I - knowingly or unknowingly - have developed an identity as being almost exclusively gay because that is just easier to manage and defend in public, especially in the west where we insist that there is a clear and consistent link between your public/family sexual identiy and what you actually are doing in bed. In SE Asia, they tend to be a little more willing to split up the public/family image (cultural institutions) and the bedroom reality and human sexuality (bodily physics, attraction, fun etc ) outside the gay/straight categories.