If you love a Thai woman, you must love spicy food!
The culture, what to expect
Thai culture, as with most Asian cultures, is drastically different from American culture. Some American men go to Thailand however, with a set of unrealistic expectations. Here are two things not to expect: Don't expect her to be your maid, and don't expect to Americanize her.
Thai women are more traditional than Western women. They are excellent cooks and good housekeepers, excellent mothers, and they respect their husbands and their families. They will surprise you, delight you, and will treat you like no American woman ever would. But--don't go there expecting to bring back a domestic servant. Thai women want to be a wife to you, a loving companion and an equal partner.
One of the most unfortunate scenarios is when an American man brings back a Thai woman, and then expects to "Americanize" her. I have heard of Thai women who are forbidden to cook Thai food, forced to abandon their Buddhist beliefs and required to foresake everything that is Thai. It just doesn't make sense! If this is what you want, then there is really no reason for you to go to Thailand. If you want to Americanize her, just marry an American woman to begin with. If you marry a Thai woman, enjoy the differences in culture (and learn to love spicy food!). Vive la difference!
Here's a few cultural tips:
Picking up the tab at dinner. The custom is usually for the eldest member of the group to pay. The American custom of arguing over who gets to pay the bill isn't usually done here, grabbing the bill out of somebody's hand may insult them. However, as a foreigner, you walk a fine line. You may still be expected to pick up the tab even if you are not the eldest. If you're out with your girlfriend's family and her dad (assuming he's older than you) wants to pay, allow him to do so. But if he doesn't reach for the bill right away, feel free to take it.
Here in the U.S., we tend to just take the bill, look at the amount and pay it. It works a little differently in Thailand. It's very likely that even if you are going to pay, somebody else--your girlfriend, her mom, her sister, etc.--is inevitably going to take it away from you to check the math before you pay. Every plate will be calculated, and there will be discussions about "did we order this," "what was on this plate," and "how many bottles of Singha did we order." Once they have been satisfied that the wait staff has not tried to cheat you, you will then be allowed to pay the bill. Tipping in Thailand is usually not done except in Western-style restaurants, but leaving the change from the bill is always welcome.
If your girlfriend refuses to hold hands with you in public, and moves away if you try to give her a friendly kiss before you put her into a cab to send her home, it's not because she doesn't love you. Showing affection in public is just usually not done, at least by "good girls." Thais and farang alike often make the incorrect assumption that, if they see a Thai woman holding hands or kissing in public with a Western man, then she is a prostitute. A "good girl" doesn't want to create this perception, and that's why she doesn't want you to get too close in public. It doesn't mean that Thai women are not affectionate, quite the contrary. They are--just wait until you get her home!
When we were dining at a restaurant in Bangkok, my Western friend who had joined us remarked about how young the waitress looked, wondering aloud to us how old she actually was. My wife innocently waved the waitress over and quite directly asked her her age. Not something you would do in the States, but generally accepted here. Asking somebody their age, career, salary, and other personal information is common.
It goes without saying, always remove your shoes when you enter into somebody's home, or a temple. You usually don't need to take off your shoes when you go into a shop, but some offices keep the tradition. Just look for a pile of shoes at the door, if you see that, remove yours and add them to the pile.
Don't sit with your legs crossed, and don't point your feet at anybody or at an image of the King or image of the Buddha. Never touch another person with your feet in any way. Don't point or gesture with your fingers.
You will see Buddhist monks everywhere. Be aware that a woman may not touch a monk and may not sit next to him. If you are on a bus for example, and there is only one vacant seat and it is next to a monk, you may sit down but your girlfriend may not. If your girlfriend observes the custom of offering gifts of food to monks in the morning, she may hand the food to you, and then you in turn will give it to the monk.
Whether or not you are a Buddhist, keep in mind that you are in a Buddhist country and have respect for the religion. Do not be disrespectful to Buddha images, don't climb on them or sit on them to pose for pictures, and never, ever use a Buddha image as just a decoration or ornament.
Thailand is well known for its beautiful beaches, and some Westerners have spoiled them for the natives by taking the attitude that nudity on the beach is acceptable. It is not.
Thai people are remarkably patient and calm. When you're in Thailand, don't worry about time too much. Bangkok traffic is the worst in the world, yet there is no road rage. In someplace like Los Angeles, if traffic is backed up and somebody in a car offends somebody else in some way, chances are, they will be given an obscene gesture, shouted at, or possibly even shot at. In Bangkok, if you're stuck in traffic, you just turn on your car radio, buy some food from a street vendor and wait it out. Example: I was in a taxi in Bangkok going to a dentist appointment, and got stuck in traffic. The time of my appointment came and went, and we were still there, with the taxi driver happily chatting away and asking about where I was from. I was getting more and more upset, looking at my watch and thinking about having to pay the dentist for a missed appointment. My wife reassured me and explained that although the dentist had said 1:00, she really meant, "any time this afternoon."
Of course, the ubiquitous "wai" is the standard greeting. This is a gesture where you press your hands together and raise them to your face, and bow slightly. Use it instead of shaking hands. There are rules about who wai's first, according to status. A woman will wai a man first, a younger person will wai an elder. Always wai your girlfriend's parents, even if you are older than they are. You usually return the gesture when somebody wai's you. However, when store clerks, waitresses, and servants wai you, you do not need to wai them back. Just nod your head slightly in return.
Thai and Western cultures are literally a world apart, and sometimes, the only exposure a Thai woman has to American culture is from television. Don't be surprised if she thinks all of America is like it is in the Hollywood movies, where everyone has a big house and money to burn. My wife, before she came to America, had never seen the type of small, crackerbox houses that were mass-produced in the post-war period , and was absolutely appalled when she saw the one I had bought for us. She had expected something a lot grander, and with two stories (we later bought a historic Victorian home about three times the size of my old crackerbox).
Be sure to be honest about your own financial situation, and don't put on airs--a very common misunderstanding will occur when an American man will come to Thailand on vacation. Naturally, since he's on vacation, he's got a pocketful of money and is free-spending. But when he meets a Thai woman and falls in love, she doesn't realize that the big spender image is just a temporary situation, and he's really just a working stiff with bills to pay and a mortgage.