This seems to be a hot button issue this year. What is marriage? I think if you ask a hundred people you will get a hundred answers. This whole issue has some serious equal protection ramifications but I want to focus on some other issues.
This whole issue is about money and financial protection.
The government has historically been in the business of encouraging behavior and I see this as more of the same. Lets take a quick stroll through the tax code for a few examples.
Marriage penalty or bonus? There has been a lot of heat about this, but this is how it worked. If you are married and only one spouse works there is an extra deduction. Reason, the government wishes to promote the traditional family model of one person working and one person staying at home with the presumption of children.
Earned income tax credit exists to subsidize the rearing of children. The cut off for a certain dollar value is done on a graduated system. So you can earn in excess of 100,000 a year and still get some form of earned income tax credit for children. This promotes the government policy of being family friendly.
Home tax deductions? The government allows people to write off the tax and interest they pay on their homes? Why? The government has made a decision, with the help of the real estate lobby, that people who own home are better Americans. Result, people are encouraged to buy homes.
As far as the tax code goes, those who get married, buy homes and have children are good Americans and those that do not are not. This is just an example, the laws of inheritance, child custody and property ownership are similarly influenced by similar policies.
Realistically, those that want same sex marriage are interested in the trappings that accompany the status of being married. These rights include survivor rights, rights of inheritance, tax credits, property ownership and health insurance. A quick example, when a married couple buys property in Pennsylvania they own it as indivisible 100%, where as, if two people buy property that are not married they own it as a divisible 100%. So, in the same sex scenario, if same sex spouse #1 (s1) and same sex spouse #2(s2) buy property, S1 could sell his/her 50% stake to another party without the consent of S2. In Contrast, in the traditional married couple example, H could not sell his interest in the Property without W doing the same and any attempt to do so would be invalid. This serves as a protection to the married couple that does not exist for others that are not married.
Moreover, as we all know the passage of property without a will is written to first pass to a Spouse and then to Children absent both it goes to Parents. (With a will do what you will but you can't ignore your spouse, they get a statutory minimum no matter what). Let's go back to our characters again, S1 dies without a will and no children all of S!'s property goes to his/ her Parents absent a will. W dies and writes a will leaving nothing to H, even if she bad mouths him saying what kind of evil person he is, he can still get 50% of her estate in most jurisdictions. This is a protection to the surviving spouse, written mostly for the protection of women it is now generally read to protect the interests of both spouses. Why not just leave gifts prior to death, bad move. A gift prior to death is given at the fair market value of the gift. (stock bought for 100 a share and given when the value is 500 means that tax is due on 400 per share, if passed at death no tax is due.)
So like most things, a lot of this issue is about money and financial protection. That doesn't make it wrong. But same sex marriage does go against values that are being promoted throughout the 50 states and at the national level. The codes are written to reward people for doing things the "right" way. Same sex marriage flies in the face of these values so it is not surprising that the federal government would be opposed.