The definition of marriage in DOMA singles out one type of marriage union, that of a man and woman. The union of a man and woman is also the only natural biological way people can reproduce. It takes a man's "stuff" and a woman's "stuff" to conceive a child. It is also the sexual behavior encouraged by majority religions. However not all religions solely recognize men & women. Other religions have vague definitions of appropriate sexuality or address "proper sexual behavior" head on. These are religions that focus on the spirit rather than the organization of the religious congregants. It is the classic "spirituality religion" versus "organized religion" clash.
This spiritual church versus organizational church clash is where it bigotry in religion falls apart. While not everyone fits in to nice neat spiritual boxes, the organizational church wants people to fit in to their rules. Even those that admonish that "God works in mysterious ways" look for people to act and behave within their organizational rules. It makes "running the church" easier for those who can't, or chose not to, acknowledge conflicts arising from the spiritual teachings and maintaining an orderly organization.
This definition of marriage in DOMA establishes a respect for the "organized religion" while prohibiting the free exercise of "spiritual religion." Period.
The antagonistic alarmists love to raise questions about what else might be covered as "free exercise of religion", but all I hear is people who want to keep their behaviors in line with what an organization has told them.The antagonists are looking for proper ways to behave and can't fathom how other people can go through their church and come up with different answers. They want to stick their noses in other peoples' business and then try to force those people back in line with the organization religion. They don't consider the spiritual pain or anguish they inflict on others just to keep their organization religion in their sense of order.
This need to control the behavior of others is the threat. Going beyond my understanding of what God, or however I define a greater power than myself, and defining how I should behave according to someone else's definition of God. Placing the outside influences on myself above my own understanding derived from my experiences, thoughts and understandings.
Antagonistic alarmists are afraid that their attempt to force heterosexual marriage on unconvinced groups of people will result in a backlash. Since they used force to try to achieve their goals, they are afraid that they will be forced to engage in homosexual behaviors regardless of their beliefs. They are afraid that they will have to take their own "medicine" as it were.