Its happened a lot to me rasg. People have asked me as if its the most normal thing in the world until I remind them that "thats my wife and mother of my children your talking about" which is usually met with a lot of spluttering about "not meaning it like that" etc.
I have also been asked it by more hostile people who have either been drinking or are inherently racist. These are the ones where you have to be more careful with your response/not respond at all as its likely to get real nasty.
As for the "Thai Bride" phrase, I have found it is said mostly by people who I havent seen for a long time it will go something like this "ere, so and so said you got one of them Thai Brides". It's usually said with a sneering undertone which I don't like.
I have often wondered if being approached about this is directly attributable to your socioeconomic status & if age difference between husband and wife makes it better/worse.
I work in construction and in this particular industry there are a lot of racist, unfiltered, poorly educated older men.
Compare this to a solicitors office for example and I would imagine there is quite a contrast in attitude or at least enough politeness to not spout their views.
One of the worst examples of this was when I first arrived back at work after picking wifey up, there were about 10 people stood talking under a scaffold. The oldest bloke shouted at me "I hear you've brought one of them F****** w*** back" which provoked raucus laughter from all of them. Luckily I dont work with any of them morons anymore.