In the latest case, legally married Edie Windsor protested the fact that the IRS charged her wife's estate over $300,000 dollars in estate taxes that would not have been charged had she been married to a man. Edie and Thea were together for 44 years.
The Federal District Court in NY found no reason why Edie's and Thea's marriage should be so disadvantaged or how this disadvantage would somehow promote straight couples marrying or bearing children.
That brings to 5 the cases that have found DOMA unconstitutional. The furthest along is in the 1st circuit (where the 1st circuit appeals court just found DOMA unconstitutional). There are 3 cases in CA (on appeal to the 9th). The current case, if appealed, will go to the second circuit.
This is headed to the Supreme Court, for sure.
Update: From the decision.
These are interests in the choices that heterosexual couples make: whether to get married, and whether and when to have children. Yet DOMA has no direct impact on heterosexual couples at all; therefore, its ability to deter those couples from having children outside of marriage, or to incentivize couples that are pregnant to get married, is remote at best. It does not follow from the exclusion of one group from federal benefits (same sex married persons) that another group of people (opposite sex married couples ) will be incentivized to take any action , whether that is marriage or procreation. …the Court cannot see a link between DOMA and childrearing. DOMA does not determine who may adopt and raise children Nor could it, as these matters of family structure and relations "belong  to the laws of the States and not to the laws of the United States."Or put another way, last night, BP and I found out the nice young couple next door is expecting a baby. "Wow", I said, "They must not know we're married lesbians!"