Actually, the fed govt DOES have a legitimate place in any lawmaking regarding marriage, for gays or not. Why? Because existing federal laws have different rules and status for people based on their marital status, as defined by both state and federal laws.
Taxes, are one example. Inheritance and property rights are others.
Here are two big example the gay marriage rights activists will (and should) use... the non-hype ones anyhow.
2 couples. Both have an adopted child, and a house with a house payment. Both work, so they have child care and retirement savings to consider. One couple is male/female, the other is male/male.
First issue - taxes. The traditional family gets numerous cash benefits designed to aid the family and children. Joint tax returns, both wage earners benefit from lower tax brackets, retirement deductions, mortgage interest deductions, and child credits. The gay couple, even if they both have the exact same incomes and expenses as the traditional couple, may pay more federal income tax because they are not considered "married". This makes an easy case against the federal government for sexual discrimination.
Second issue - death.
For the traditional family, if one family member dies, the other will (in most states) receive both custody of the child and rights to all "joint" property. After a court hearing, most widows/widowers will also receive rights to all "non-joint" property that is not otherwise given to another person in a will (such as bank accounts held since before getting married).
For the gay couple, there are numerous problems that have already occurred in "real life" and which still occur. First, if the one with his name on the house title dies, there is a good chance that the surviving partner will not receive ANY rights to the home and will in fact be evicted while the state determines who gets custody (usually extended "birth" family). The same problem goes for other property, such as investments, vehicles, and bank accounts. Second, is the issue of child custody. Not only may the surviving member lose their house, custody of the child may not pass to the surviving member regardless of who owns the house. Again, this has actually happened and still happens.
Here's the kicker - most (all?) states have "common law" marriage rules that mean a couple who has never been formally "married" to be in fact married after simply living together for a number of years, especially if they show some sort of marital intent such as having kids or owning joint property. But now there is a push to outlaw gay marriage to the point where an unmarried male/female couple will have more legal rights than a gay couple holding a marriage certificate from one of the few states that gives them out.
That's F**ed up IMHO. If the govt is going to set rules designed to provide family stability and ensure consistent legal application of property and custody of minors, then those rules need to apply to EVERYONE who could possibly be subject to those situations.
Simply put:
It's unfair to provide cash benefits to "married" couples including common law couples, and deny them to gay partners who have established a legitimate intent to establish a household together. Many of those tax laws are intended to benefit children, and it is fairly common for same-sex couples to have custody of a child either through adoption or from a previous marriage or non-marital relationship.
It's unfair to deny the right of survivorship to same-sex partners who have established a household or lifelong partnership, when those rights are usually given to traditional couples who were never formally married but fall under common law marriage rules. This form of discrimination not only directly impacts any children in the family, but it also disrupts the orderly conduct of society and the rule of law, when the laws are applied purely based on if the survivor was of the same or different sex as the deceased.
My solution (of course I have a solution) is to create a new federal definition of legal domestic partnership that applies to ANY type of "marriage" relationship, and give people who gain this new partnership status the exact same privledges, rights, and benefits that "married" couples have, without exception.
Yes there would be problems to be overcome (since bigamy is illegal, what about group "partnerships" with more than one couple?) but I think the federal govt needs to do something to address what are IMHO clear legal and social injustices being carried out, without making it worse by narrowing the definition of marriage while excluding other partnerships from the legal protections and financial benefits that traditional married couples have.
It's funny/sad how the same old arguments used to discriminate against blacks are now being used to discriminate against gay couples... If they don't want it to be called marriage, fine. Abolish the whole legal concept of marriage and replace it with a legal definition of "domestic partner" if necessary, but the raw and unconstitutional discrimination against couples based purely on sex needs to be addressed at BOTH the state and federal level.