Maybe you didn’t want to offend; you were only asking questions to be polite. This is the Thai custom. However, the foreign custom is different. They may be offended if you ask about a boyfriend or girlfriend, because this is something they only talk about to a very good friend. The answer is in their hearts, and they don’t want to share it with someone they just met. When you ask questions that are personal and private, you are taking away their privacy, and that makes them angry.