In reality, we do not know the sexual orientation of married partners. The same is true for members of same-sex couples. The marriage discrimination faced by same-sex couples is discrimination based on their sex, not on their orientation. Lesbian or gay people can get legally married, as long as they marry an opposite-sex partner. The fight is for the same access to legal marriage as all other citizen have.
Legal marriage is a state-defined contract currently available only to a man and a woman.
Every state has a different set of laws, numbering from 160 to more than 250. The U.S. Federal system has more than 1,040 laws which are triggered by legal marriage. These laws interface with almost every sphere of social interaction. They treat a family as a legal unit, which is especially important during times of change, such as divorce, or during a family crisis, such as the right for a spouse to make medical decisions. Marriage laws allow for the understanding of the married couple as full adults in our culture.