A complete waste of time
First off, I consider the entire subject of gay marriage a waste of Congress's time. We've got far more important things to deal with than telling people who they can and can not marry. Especially when nobody is really stopping anyone to begin with (see below.)
It's about judicial activism, not gay marriage
If this entire debate was about state legislatures passing gay marriage laws, I wouldn't have anything to complain about. I am a firm believer in the 10th Amendment of the US Constitution, which guarantees states all rights that are not explicitly reserved for the Federal government or prohibited from the states. And laws regarding marriage (of all kinds) is exactly such a right.
But we're not talking about states passing laws. We're talking about activist judges legislating from the bench. We've already seen one state supreme court (MA) ordering the state legislature to pass laws permitting gay marriage. The population of the state have no say in the matter. And the US Supreme Court seems to have upheld their right to do this.
The fact that the US Supreme Court is supporting this clear violation of Constitutional separation of powers means that a response is necessary. And the only Constitutional way to override a US Supreme Court decision is to pass a constitutional amendment. I would personally prefer to use some other more meaningful issue to fight the USSC over, but we weren't given that choice. MA's supreme court didn't order the MA legislature to pass any other law, so that's where the fight must be fought.
If Congress does nothing here, it sets the precedent that the country has no problem with this kind of extreme judicial activism. And the next time a state supreme court orders a state legislature to pass an unwanted law, it will be that much harder to fight the decision.
All this having been said, I don't like either side in the debate (to ban or guarantee gay marriage). As far as I'm concerned, the government should get out of the marriage business altogether.
It's state recognition, not marriage
As it stands right now, no state actually prohibits gay marriage. A gay couple has only to find an appropriate member of the clergy to perform the ceremony and they will be married under the eyes of whatever god or gods this clergyman represents. And there is no state law that prohibits this. While there are many religions that prohibit gay marriage, there are also plenty to permit it, so this is not a difficult thing for a couple to do.
In other words, the entire fight is over state recognition. I say "who cares?" Or more explicitly, why should anybody, whether gay or straight, care about the state recognizing their marriage. Marriage is a covenant between two people and whatever god or gods they believe in. Or in the case of atheists, just between two people. Why should I care what the government thinks? And why should the government care about my marriage?
In the US, we have separation of church and state. This means God's laws should have nothing to do with the state's laws. If you reject all arguments based on religion, what is the difference between a marriage and a "civil union" or "domestic partnership"? Absolutely nothing!
Government gets involved in things like taxation (allowing a couple to file jointly), settlement of property after death/divorce/separation, and powers of attorney. All of which can be accomplished without any concept of marriage, using existing laws!
For example, the first two can be covered by existing corporate partnership laws. If two people create a partnership business, and give all their assets to the partnership, they end up paying no individual taxes, but the partnership files corporate taxes on its income. If the partnership is dissolved, the partnership contract will contain the terms of property distribution (or laws will define these terms if the contract doesn't have anything.) Similarly, there already exists laws where one person can grant power of attorney to another, to be used in case of mental or physical incapacity.
Note that the sexual preference of the people involved in this scenario are completely irrelevant, because we're now dealing with terms of incorporation and not "marriage". This system would be used for traditional heterosexual partnerships as well as homosexual ones. The whole concept of "marriage" ends up solely as a religious one, having nothing at all to do with government.
I'm sure some will complain about this, claiming that it promotes polygamy (since contractual partnerships can have more than two partners.) But this isn't true. We're talking about a partnership in order to gain specific financial/property benefits, not granting rights of sexuality. The right to enter in a group-sex relationship remains just as legal/illegal as it is right now - in actual practice, nothing changes.
And lets face it, legal or not, there are groups of people today that have group-sex relationships. My proposal won't create them, ban them, or support them. The same things that make them unpopular now (religious and family pressure) will continue to make them unpopular under my proposal. And when society changes (as it always does) to make them become more or less stigmatized, my proposal will have no effect, and will not be affected by these changes.
My proposal also stops the slippery slope of people wanting to demand other kinds of "marriage" rights - like to children and animals. (Yes, there are groups that advocate this, and I'm certain some of them will become much more vocal once gay marriage becomes the norm in society.) Under my proposal, these kinds of partnerships can't happen, because children and animals can not legally enter into a contractual relationship of any kind.
It also eliminates the pressure from those groups that would like to promote incestuous relationships. Since these partnership contracts are purely social/financial, it's not an issue. A brother and sister can choose to enter into a partnership together if they really want to.
This all works because these partnerships don't grant any kind of sexual rights/privileges. If the partners want to have a sexual relationship, existing laws already exist to define what is allowed or prohibited. Incest can remain illegal, even if the two people are contractual partners. And religious organizations can retain the moral and social authority they currently have over their members.
I'm curious about what you think about this proposal. I think it solves a whole world of problems. It strengthens the separation of church and state. It takes the issue away from the judicial activists. And it is flexible enough to not require reorganization as social norms change. And government doesn't have to take sides in the debate over people's sexuality.
I'm certain the polititians and judges would uniformly hate this proposal, because it takes power away from them both. But I'm not trying to placate government. I'm trying to solve what I see as the real problem here.