Because all citizens pass through the institution of the family, which is the bedrock of every society.
Moreover, the State must arbitrate all family disputes, including adoption, child-custody, etc.
Ironically, state sanctioning of homosexual marriage is suicidal. No procreation, no society, no State.
The gist of your argument seems to be "for the children". The state does everything you say now, regardless of "marriage" which is in the realm of religion not Government.