DOMA is completely constitutional. There's nothing in the Constitution that prevents it. Further, the government does have a right to promote good behaviors that lead to success, health, and prosperity, though not necessarily through law alone. That being said, DOMA is really an attempt to check deviancy from being redefined.
The issue really comes down to two things; does a distinct minority get to force the rest of society to accept their deviant lifestyle choices by redefining marriage, and what role does government have to promote a free society from descending down into permissiveness and licentiousness?