HKGUY - That seems like it would be a pretty hard law to control though. The onus would be on the police to prove someone married JUST for visa rights and that the couple didn't love each other.
It would seem to be extremely hard to prove or disprove love, and it's not something any country's government has any real moral right to do.
Which basically means a government would only even start to bother if something seemed like a very suspicious marriage of convenience. And the police/governments risk getting themselves in trouble for racism in that area - as it seems, in the west at least, most of these cases come when middle eastern/filipino/african people marry western people, and it's not exactly impossible for love to be involved.
In other words - if you marry someone you love you don't have anything to worry about.