You've got it backwards. In rational basis review, the law being challenged is presumed constitutional. The party challenging it has the burden of showing that is is not rationally related to any legitimate government purpose.
That is usually very hard to do, because under this standard courts defer strongly to the legislature that passed the law being challenged. They do that to respect the basic principle of the separation of powers that's built into the Constitution. That requires courts in equal protection cases to have a damn good reason to substitute their judgment for the legislature's--and indirectly, for the judgment of the people who elected that legislature.
Take public nudity as an example. If a state law prohibits it, or allows it only in a few specified places, is it denying nudists the equal protection of the laws in violation of the 14th Amendment? If some nudists' rights group sued, would the state, to justify its law, have to produce evidence that allowing public nudity is an unwise, socially destructive policy? Would it have to refute evidence presented by the nudists that its view was unfounded, because public nudity has never been shown to have harmed anyone where it was practiced?
Of course not. Ordinary rational basis review would apply, because there is no fundamental right to go nude in public, nor do laws that discriminate against nudists create a suspect classification. The nudist group wouldn't stand a snowball's chance in hell of winning.
What bigotry! How terribly cruel and unfair! What about their rights to live as wonderful, loving people who mean no harm to anyone, free from oppression? Why doesn't the Supreme Court Do something!!??
Maybe next we'll see nudists recognized as a new victim group, trying like the others, no doubt, to trade on the imagery of the black civil rights movement. I can picture them marching--in the snow, maybe, to symbolize the suffering they've endured all these years. "We shall overcome some day-ay-ay-ay-ay . . ."