Marriage preceded the state. The state chose to recognize marriage as a legal contract for a variety of reasons, few or none of which are applicable in a wider sense to homosexual unions. Moreover, marriage is defined as a heterosexual union.
Some of the reasons the State's interest is or was in recognizing marriage are:
children and heirs
protection of women/Moms
property rights and dispensation
Another valid reason for the State to recognize marriage is that marriage is the initial contract which serves as the primary basis for the formation of the family. Not saying there are not families without marriage, but marriage has been the standard and has helped to establish certain principles for the formation of the family such as the ones mentioned above.
It's important to realize that marriage and the family precedes the State. A big problem in this debate is that marriage is being redefined in order to view it as an individual right rather than a familial contract which the State honors. Marriage is not a right for individuals but something the State recognizes as the basis for the formation of families, child-rearing, etc,.....The idea that homosexuals deserve this "right", imo, stems partly from their desires to have their unions legally and socially sanctioned.
That may be an emotional need but it misses the point on what marriage is and what the State's interest is. The State should not be in the business of making an ethical decision contrary to the majority's wishes to promote homosexuality as normative. The State had valid reasons for recognizing marriage and may have valid reasons for recognizing homosexual civil unions since some injustices such as a partner not being able to make health decisions or visiting rights in hospitals, etc,....
But there is no compelling reason for the State to accept homosexual unions as marriage and give them equal status under the law. There are several reasons for this:
1. Despite many children in homosexual union households, it's not like homosexual sex naturally produces children which need legal status conferred by marriage. With DNA, the fear of bastard children (not meant in a derogatory sense but literal sense) without soceity being able to know the identity of the father is lessened, but it's still a real point.
2. The State's interest is in promoting marriage between a man and a woman for children. That's one reason the State recognizes marriage. It is true that some homosexual unions are probably better than heterosexual marriages for some children. People are different and there are always exceptions to the rule. But the idea is that women have babies. Haven't figured out to get around that, and it's better for children to have Dad around. Sorry if this sounds hateful to lesbian couples or whomever, and I am quite sure many do a bang-up job as parents, but the State cannot afford to micromanage families and we have a long history of promoting Mom and Dad, not 2 Daddys or 2 Mommies, and the arguments we should change this standard don't measure up. The interest of the family and the children, statistically overall or just what we think intuitively and has worked, trumps the emotional needs of homosexuals to feel accepted and beloved by soceity.
Could say more but the above 2 points are probably sufficient.