The history of litigation surrounding equality issues often involves the unexpected scenario of rights expansions only occurring when dominant group members claim discrimination under a law. The best example of this is pregnancy leave- women are often not guaranteed sufficient amounts of paid leave to take care of children post-pregnancy. When fathers who wished to stay-at-home with the children after childbirth litigated for paid leave, the courts finally realized that this was an equality issue.
On the gay marriage front, cases like Goodridge (MA case allowing gay marriage) and Perry v. Schwartzenegger (the CA case striking down an amendment defining marriage as between a man and a woman) have noted that, as with racial discrimination, “separate is not equal.” The Courts noted that such laws create a separate institution (Civil unions) for gay people, for the express purpose of denying them the ability to marry based on their choice of partner. Now, a lawsuit in the U.K. by straight couples who were not allowed to obtain civil unions offers a chance to test the theory that courts can only see this type of discrimination when members of the “in-group” challenge it.