In short, marriage is still important in western civilization to ration females in equal share for everyone and hence ensuring votes from singles in coming election. Other importance of marriage has gone.

Now for the explanation:

Anything created is there to max out the interest of the creators. The same way, marriage laws are there to max out the interest of those who make marital laws. That is, whoever are in power of the society.

Furthermore, is marriage nothing more than a tyrannical institution designed to control populations?
Yes. Not much more now.