This is a tough subject. You're right that if you walk around a city in SE Asia, you'll inevitably see lots of old, fat, bald men walking around with young pretty local girls, and there's no mistaking what's going on. Lots of these guys marry their bar girls and bring them home, so it's not uncommon to see a surly, tattoo'd guy walking around in Manchester with a Thai girl who's clearly way out of his league.

But what happens if you're a young traveler living in Chiang Mai, teaching English, and you meet a local girl who works as a bank teller. You fall in love, meet her parents, marry and eventually move back to your country with her. How do you signal to the world that yours is in fact a legitimate relationship?

The short answer is: you can't. There's a stigma, deserved or undeserved. It's something you learn to live with, because you know that nomatter how clearly you explain your position, you'll still end up fielding uncomfortable questions like the one asked by the parent.