The U.S is (amongst others) founded on the principle of separation of the state and church. This has been interpreted to mean that the government should not "prefer one religion to another, or religion to irreligion". One can argue that this principle is not always followed, as perhaps most obvious when looking at our dollar bills that state "In God We Trust", a slogan that is also the national motto.
A less visible promotion of religion is perhaps the way the income tax structure is set up in this country. It seems there is an inherent promotion of marriage and consequent penalization of civil unions or co-habitation couples in the structure of exemption categories.
Couples living together without a marriage license at most qualify as "head of household" thereby excluding them from the exemption allowed married couples. For the purpose of federal taxes civil union partners will usually file as "single" on their federal tax returns. Another benefit granted married couples is that one can claim a spouse as a dependent thereby making employer-provided benefits such as health coverage a tax-free benefit. For domestic partners, such benefits are instead regarded as additional taxable income for employees.
A less known (at least for me) benefit is that as a part of a married couple one can create a family partnership, allowing you to divide business income among family members.
This is an issue that is broader than whether or not same-sex couples should be granted equal rights, it goes to the very core of the principles this country was founded on. The promotion of marriage is in many ways the promotion of religion and as such, a clear breach of the 1st amendment and the principle of separating state and church.