The only way offspring is produced is through the union of man and woman (even when done artificially in the laboratory).
The State has an interest in promoting the good of offspring, irregardless of what Ron Paul thinks because it benefits society with new members.
The Church too is interested in this offspring because they are destine for eternal life.
All Christian denominations as well as the public at large accepted this view of Marriage until the 1950's. You got married in order to start a family.
Society at large and the Church accepted this union and conferred special benefits because you were helping to populate earth (for civil society) and heaven (for the Church).
Once children are removed from marriage through contraception and abortion, than the equation changes.
There is a greater distance between heterosexuals trying to conceive and those blocking it, than there is is between heterosexual blocking conception of children and homosexual unions. That former is open to offspring, the latter is only about sex.
If you are an atheist, than nature tells you the biological the purpose of sexual union is procreation. The very fact that every personal on this earth scientifically and biologically shows this is sufficient.