Traditionally when government has a vested interest in certain behaviours of its constituents, it may at its own discretion choose to reward such behaviours through tax incentives or other special privileges. Those who serve in armed conflict are entitled to certain veteran benefits. Granting such benefits to veterans is not regarded as act of discrimination against non-veterans. Rather it is a token reward for serving the government in a necessary and vital capacity. The government necessarily encourages participation in the armed forces expressly because such participation is vital to the nation’s national defence. The government may also reward other less heroic behaviours. Tax incentives are often given for home ownership. This is not an act of discrimination against renters. Rather it is in recognition that home ownership generally enhances the stability and welfare of society. When the government has a vested interest in certain behaviours over other behaviours, it has the right to reward those behaviours.
Does the government have an inherent vested interest in heterosexual marriage over homosexual marriage? The simple answer is “yes” for two reasons. First, new citizens are born out of the expressed heterosexual marriage in a way they are not produced from homosexual marriage. The future of the next generation of tax-payers and soldiers depends on procreation, and the government has a vested interest in encouraging children being born into an accountable family unit. The second benefit provided to the government (in both traditional and in modern societies) is a presumably automatic identification of the father responsible for the welfare of the child. When it works as intended marriage provides immediate social recognition of which man (the woman is obvious) is responsible for which child. When it doesn’t work as intended and children are routinely born outside of wedlock, the government is required to be much more involved in the legal entanglements of identifying which adult is responsible for which child. Homosexual marriage provides neither of these benefits to the government since it is by nature an infertile relationship. Though not every heterosexual marriage produces offspring, it is heterosexual marriage alone that is capable of providing an environment for procreation that is optimal for the future welfare of the newly born citizen. When homosexuals or other citizens choose to adopt and raise children, it is right and appropriate for the government to give them benefits (tax credits, etc.) for doing so. But the unique benefits that accrue to the government from the committed heterosexual relationship makes it also worthy of formal recognition and certain distinct social benefits. It is these benefits societies traditionally have socially recognized and granted under the rubric of the special designation “marriage”.