In equal protection challenges, the Supreme Court applies its "strict scrutiny" standard only when evaluating government action which involves either a fundamental right or a suspect classification of persons. The Court has recognized only three such classifications: race, national origin, and possibly alienage. When the classification would burden a person because on his status as a member of a racial or national origin minority--and possibly as an alien, although the Court has not made clear that strict scrutiny applies in that case--the government action is almost certain to be invalid.
The Court also recognizes a couple "quasi-suspect" classifications: gender and birth legitimacy. When reviewing government action based on either of these classifications, it applies an intermediate standard of review and will invalidate the action unless is is substantially related to an important government interest.
If any other classification is involved, though, the government action will be upheld unless the challenger proves it is not rationally related to a legitimate government interest. So, for example, in Nordlinger v. Hahn, 505 U.S. 1 (1992), the Court upheld a California law under which the petitioner had had to pay several times as much property tax as her neighbor, even though their houses and properties were very similar. The law's basis for this unequal treatment was simply that the neighbor had been there for some years and the petitioner was a relative newcomer.
Throughout the U.S., many tens of thousands of laws discriminate against this or that class of persons in various ways, and yet are completely constitutional.