This question seems to escape most people, who can't see the forest for the trees. The ONLY legitimate government interest in civil marriage is the protection and welfare of children. That is why the marriage "contract" was invented; to create a system of rewards and responsibilities which encourages stable family units.
Unfortunately, this system has been compromised over the years by people who want enjoy the rewards without the responsibilities (no-fault divorce being a prime example.) Combined with a rising illegitimacy rate, this is making a mockery of marriage. Is there any reason to be married over than perceived tax benefits?
Ooh an idealist.

Unfortunately, government interest in marriage has nothing to do with the welfare of children, but rather the welfare of the state itself. Marrieds have economic and legal adjustments bestowed upon them. To do those, some legal definition of what it means to be married must be established so single people aren't getting tax breaks for marrying their imaginary friends and such.

Has nothing to do with the welfare of offspring though. The laws governing when teens can have sex aren't about the teens having sex but the concern over single unwed teenaged mothers which are an economic drain on the state since they usually end up on welfare.
Proof of concept:
CALIFORNIA PENAL CODE
SECTION 261-269
"WHY SO MUCH ATTENTION TO STATUTORY RAPE?
California has the highest teen birth rate in the U.S.
Every 8 minutes, a teenager in California has a baby.
3 of 4 births to High School girls are fathered by adults.
Men over 25 account for twice as many teen births as boys under 18 years old.
The Average Age difference between the teen victim and the adult defendant in cases filed by the District
Attorney is 7years 9months.
Men over 20 are responsible for 5 times as many births among junior high school girls.
In California almost 70% of teen births are fathered by adult men.
On an average California day 76 teenage girls, 17 & younger, will give birth.
In Stanislaus County over 6% of teenage girls will give birth in any given year.
In 1993 1,572 births in California were to mothers 14 years or younger."
Laws almost NEVER come about out of purely noble intentions. It's almost always about money.