Ideally, the purpose of a government is to make sure contracts are enforced, protect people's property and financial interests (the point of preventing illegal immigration), and to ensure that people have the ability to be healthy (making sure everyone has access to food, housing, healthcare, clothing) and safe (preventing people from hurting each other).
That's it.
Just because the government does more than that, doesn't make it right. (You'll find very few people who agree with everything their government does.)
By the way, I deleted the latter part of my previous post because I agree that as long as the government is involved in marriage, then it does have the right to restrict marriages to an extent, and the idea of both people living separately is a bit too much. However, the government does not have the right to judge whether or not two people should be married based on whether or not they are in love with each other. That's way too personal.
Once again, the government has the right to make a buck. It does not have the right to make a buck in an unreasonable or unfair manner. Those of you who think the government has the right to take extra money simply because it needs it, why don't you donate an extra hundred pounds to Inland Revenue?