Sorry if I'm preaching to the converted (maybe of use to someone who is not versed)...
I think it depends on a lot. Some people will see it as Santa and the college girl (pic above), some will be more open minded. Their minds will be swayed by how they recognise her and how you react together. In the above the guy is dressed like a tourist (white legs and shorts a dead giveaway) - and a poor one at that (whether he is or not is beside the point), they are holding hands in public, there is a big age gap, she is wearing very short pants. To most Thais she would be assumed to be a hooker and he a sex-tourist.

Show respect for your girl when you are in the street - i.e. walk side by side, but no kissing stroking hair (or face or other body parts) and hand holding is better left to private if it is likely to be misinterpreted. Try to ensure she dresses in an 'acceptable' way - I have a friend who is married to a Scotsman. They are a nice couple. The live in the UK. In the UK its fine for all the above, and she always dresses prettily and makes herself up - she has a clevage and shows it. In Thailand he dresses like a tourist (he is) and she wears the same low cut tops, hot paints etc. She has never been a hooker and they are perhaps 5 years apart in age - yet she has had many comments and much disdain in Thailand. Once they took a taxi and the driver tried to overchage them - the driver assumed she was a hooker and the farang could afford it - when she argued about the cost, he said as much and she was offended. The old saying goes - when in Rome.