Curries are most of the time pre-prepared, but most Thai food is prepared as you order. So it is never a problem to ask for “Mai Pet Maa”, Thai for “not too spicy please”. Even many Thai cannot or do not eat the very spicy food.
Better is to train yourself a little bit into more spicy food :-). You will discover after some time (you need to get used to it), that it works like a stimulus for more intensive taste. And chili helps your body to cool down in the Thai heat. It is good your your health and does not harm you in any way. We should all eat more spicy food.
If you do not like the spicy stuff? Do not worry, there are many non-spicy dishes as well and just remember always ask for “Mai Pet Maa”.