In Western countries, public displays of affection or PDA are very common. It's considered normal behavior. People can hold hands and kiss in public. However, Thai culture is very conservative. In Thailand, public displays of sexual affection are unacceptable, especially in some areas. Anything more than a hand hold or linked arm is just a poor show.
Thai people extremely care what others think. In order to treat properly and politely, a man should not touch a woman. No Thai women feel comfortable with the public displays of affection. Kissing in public is seen as shameful and inconsiderate.
To avoid judgments at all costs, Thai woman expect a Western man to walk a few feet away in public. It doesn't mean she has lost her interest but this is very important for her not to be seen as a bar girl.
Here is an opinion about the PDA from a Thai woman as an outsider:
"I think the Thai's general view is that these activities are reserved for the bedroom; not the skytrain or shopping mall. I agree with this thinking as well. When I see a foreigner on the skytrain with his Thai girl and he has his hands all over her ass and he's pecking at her ears and lips, I mentally label her a whore. I'm sorry, but that's what has been culturally ingrained in my head.
"When I see a young Thai couple doing this, I think...hmm, they are the newer generation of 'Siam Square trendsters' who have disregarded their social mores and don't feel shy anymore."