If you want to defend something, you have to know what threatens it. Marriage is a very dynamic institution based on love and deep commitment between two people - yes, in the tradition Indo-European cultures, it has thus far been between a man and a woman. But in order for something to threaten marriage - hence it requiring a defense - that something would have to threaten the bedrocks of marriage: love and commitment.
Gay couples, like their heterosexual counterparts, love each other, and can be deeply committed to each other. That fact is not in question. And by that virtue, it cannot threaten marriage, since gays don't threaten the basic foundations of love or commitment.
What does threaten marriage, then? Inevitably, the answer is, whatever threatens love and commitment. What can threaten love? Commitment? Several things: first, stress. Today's families are increasingly squeezed, are working long hours and overtime, and still in debt up to their eyeballs. That stress devastates people. It takes away love, commitment.
Second, lack of heathcare. When couples work overtime and still cannot pay for their or their kids' health insurance, and their kids end up in hospital, it tears the very fabric of a family apart. Good health keeps love and commitment strong, and it makes it possible for people to fill their commitment to each other.
3rd, lack of understanding about family, how to plan a family and its future, and how to plan for parenthood, if one chooses to be a parent. If a married couple does not understand that a family is a joint unit, a joint responsibility, an institution of respect, and can be made better with planning, the marriage often fails.
4th, and perhaps most importantly, domestic violence. I don't think it needs to be explained how it threatens love and commitment.
5th, loss of trust. If a couple cannot trust each other, they can no longer have commitment to each other. And there are lots of other things.
Now, the state (government) cannot do much about internal trust in a couple. But they can do an awful lot for the first four situations. Stress is often result of an overdemanding work environment that pays too little. Institute family leave and a LIVING WAGE to deal with that. The solution to the 2nd threat is obvious. Universal healthcare. Just do it. The third actually requires some understanding and letting go of ideological lines. It means funding family planning services - including planning pregnancy, and yes, abortion - for poor couples. Reproductive freedom - including contraception, condoms, and abortion as well as adoption options - needs to be a central part of that family planning. The other side of the planning will have to include funding for poor couples to be able to get low-cost or free financial planning advice. It does not require a big government bereaucracy. Each community can do their own services if the funds are available. Solving the problem of domestic violence requires first all of the above, and more. It in addition requires strict law enforcement and swift prosecution, funding for community counseling services, for early childhood intervention to prevent child and child sexual abuse, for safe shelters for the abused and their children, and better spreading of information about what to do if one (man or woman) is subject to domestic violence.
So, let's recap. The answers to protecting marriage are: family leave legislation and a living wage, universal healthcare, funding of family and financial planning services for the poor with full range of reproductive freedom, fund early childhood intervention, family counseling services and a strong law enforcement effort against perpetrators of domestic violence, and free rein of information.
The defense of marriage against the forces that threaten it is solidly progressive!