Adam Kolasinski wrote The Secular Case Against Gay Marriage in The Tech. His argument is that heterosexual couples receive subsidies from the government because they provide an important service for society, namely procreation. Homosexual couples cannot procreate and therefore should receive no subsidies. If homosexual marriages were legalized, more couples would need to be subsidized, which would add costs to the state without any increase in benefits in the form of more baby production.
If heterosexual couples are subsidized by the state and if homosexual couples are also subsidized, indeed public expenditure would rise.
The problem with this argument is that is assumes that subsidizing heterosexual marriage causes an increase in the rate of baby production. Perhaps most couples who produce babies were already going to produce babies and the presence of subsidies for marriage does not affect their decision to produce babies. In Australia, the government paid $5000 to women who had babies. The rate of baby production seemed to go up from 1.7 babies per woman to about 1.9 babies per woman, but this is still far too low and may have been due to random movement. Most couples who receive the $5000 from the government for having a baby were going to have a baby anyway, so the money spent trying to bribe couples into having babies was wasted. It is widely agreed that this policy of paying women to have babies, called the Baby Bonus, was a failure (read Coming Out Against the Baby Bonus). If paying women to produce babies does not work, then why would we expect the rate of baby production to increase if there are payments to heterosexual couples in the form of subsidies to marriage?
I agree with Adam that it is important that people procreate. In many developed countries today there is the danger that the lack of baby production is causing the population to age, which increases public health costs and decreases income tax revenue as the number of young people working falls.
The law of comparative advantage states that economic efficiency comes about when people specialize and then trade. Many mother today juggle baby production and baby rearing with work such as being a teacher or being a clerk. This is inefficient, according to the law of comparative advantage. What should happen is that there be women who specialize in working as clerks or teachers and other women who specialize in baby production.
The most efficient way for government to produce babies is not to subsidize marriage or to bribe mothers. Rather, the government should actually contract out the job of baby production to a private firm that hires women to be mass inseminated from semen that is collected from men for a small fee. These women are paid for their services and once the baby is born it is raised also by the firm. The government can pay the firm according to its rate of baby production as well as other indicators of the quality of the children produced, e.g. scores the child receives in English tests.
Hence subsidizing heterosexual marriage is not the most efficient way of increasing birth rate. There are more efficient ways to do it and because of this heterosexual couples should not receive subsidies.
Furthermore, we can argue why allowing homosexual marriage improves the economy. Homosexual marriage is voluntary. Two parties agree to homosexual marriage because they believe that by doing so they get more utility than if they engaged in heterosexual marriage. If not, why would they engage in homosexual marriage? If homosexual marriage is banned then the party that would have engage in homosexual marriage does not and suffers from lower utility and a trade that would have occured does not, creating an economic deadweight loss. Banning homosexual marriage is a marriage market restriction that reduces overall welfare.