In a country where gay marriage/union is legal if they travel to another country that has the marriage/union?

is their marriage still recognised in that other country if they visit or move there? Or is it only recognised in the country they got married in and becomes null and void anywhere else?

I think it's only recognized there. Let's all just move to Canada :)

If they visited, I don't think the other country could do anything about their marriage. If they became citizens in a country where gay marriage was illegal, however, the marriage might become null and void.